The Aedra were not 'weakened' by the creation of Mundus. Instead, the creation of Mundus exposed their deficiency
tl;dr ahead of time: the Aedra didn't "become" weaker - rather their temporal and non-Padomaic (Daedra embody Change and Eternity/Permanence) nature is being tested (time - which is their Anuic nature conflicting with itself, ergo Dragon Breaks etc - showcasing their weaknesses) so they can achieve 'higher being', and only their version of the narrative implies that they were 'weakened' as that one is 'temporal'. In the Daedric version, the Anu are weak and dependent on stasis while assuming 'perfection', and time 'entraps them and shows them their weakness' - they get lost in Madness (Akatosh representing the Anuic-Through-Time is said to share in on Lorkhan ergo Sheogoraths madness). In other words, the Anu are exhaustible and only do well if they can indefinitely prepare, but the more the stability of that which they 'prepared' is tested, the more it falls apart (which is manifest as the 'present' relative to 'history'). The Daedra are 'inexhaustible' 'eternal' and 'infinitely flexible' and because of that, embody the principle of metaphysical 'Buddhahood'. This is why the Daedra can easily be their own realm, while the Aedra (who may collectively be trying to 'embody' Lorkhan) who are many make up 'one' realm (some equate it as 'Lorkhans realm' - a motif visually paralleled by the Amulet of Kings 'red Jewel sorrouned by 8 small jewels') and do so with great difficulty.
...A common myth about Lorkhan, from the orthodoxy of the exoteric view is that
>Lorkhan "tricked" the Anuic soon-to-be-Daedra into creating Mundus, weakening them and binding them to it.
But this is only one side of the story. On the flip side, the Daedra version of this myth implies
>The Padomaic Daedric Princes (who embody Change and Permanency and are CHIM aka Royalty) "forced" the Anuic that assumed they were perfect "Jyggalag" into Sheogorath (who is an archetype of Lorkhanic transformation into a Daedric Prince). This way, Jyggalag was constantly aware of his flaws as Sheogorath, and the madness (which Akatosh aka the Aedra of time shares in, embodying its scope on Nirn) as it contradicted him - thus creating one of the Lorkhanic archetypes by which the Dreamer self-reflects.
A nice callback to the ideas of 'madness' and open-mindedness, a very common historic trope in Initiatory and Mystical rites. Obviously this starts with 'reluctance' due to the 'shame' of what they had witnessed, but with the moving of the Anuic pantheon to the Eight (and then Nine) they seem to become more capable of managing and accepting of the Padomaic aspect. With Arden Sul variant stories implying notions like
>Arden Sul was so happy and blissful of Mania that his heart exploded after he did Greenmote
or
>Arden Sul plucked out his heart to discover he was his own traitor, destined to take his own life
The Aedra are often considered the weaker and loftier of the Gods who are 'dependent' on Mundus. The Daedra on the other hand are the more distant, yet powerful Gods who are self sufficient, complete, Royal Princes. If you've been reading my other posts on the topic of Anu and Padomay, this should all resonate.
But the notion of this topic is the idea that the 'history' of the Aedra is from the 'Aedra' centered perspective. But that is because the Aedra embody cosmic consistency, order and stasis (and Judgement). Instead:
>The Aedra assumed they were 'perfect' and that Lorkhan tricked them. Instead, as Lorkhan brought them into Mundus to show them their own reflection, they realized just how imperfect their stasis was and began their journey towards CHIM (as being Anu-Padomaic entities at that point)
>the Aedras cognitive dissonance on the nature of their own inferiority is present itself through the history of the many interpretations of the Gods themselves, and the reason they're seemingly 'stuck' is that some of those interpretations of the self-conflicting Aedra keep them in there? Think back to 'Madness' and the head with many faces.
In other words, the Aedra are weak and doing their best, and just failing miserably, ergo why they're 'trapped'. They basically got fumbled into a cosmic egg by the Daedra so that they can grow and become more powerful to eventually become pure Padomaic entities. This 'literary egg' motif is often used for Mundus and the nature of its growth through overcoming (in part, it is a motif for Oblivion when Mehrunes Dagon threatens Mundus with a 'reset' until Mundus learns to fend for itself)
And with the inrepretation of Arden-Sul:
>The Anuic pre-Aedra had the Lorkhanic aspect thrust upon them (Anu-Padomaic self reflection and initiation) which in their confusion made them realize their imperfection and converted them into Anu-Padomaic
In this view, the Heart of Lorkhan symbolizes some kind of 'learning toy' meant for self reflection for the 'baby' Aedra Gods, and with things like the disappearance of the Heart being a tantrum of the Aedra or possibly the shame of self reflection. With the Amulet of Kings and Alduin (who was meant to end the world) it seems the implication is some sort of 'test' that Mundus is passing by 'solving its own weaknesses and external dependencies' becoming 'inherently more powerful'. It's possible that Hearts embody purpose and stability, but it is currently hard to see as to what it might exactly imply.
In fact, Dagons realm of 'restarting' and 'infinite possible RE-Begginings' is something only Padomaic nature could manifest, and would be impossible with the Anuic - ergo he makes a great 'antagonist'
In other words, the Aedra didn't "become" weaker - rather the temporal and non-Padomaic (Daedra embody Change and Eternity/Permanence) nature they have is also being tested (and the illusion of time makes them seem weaker), and only their version of the narrative implies that they were 'weakened' as that one is 'temporal'. One could relate this to the question of Talos in Skyrim - why is the Dominion trying to push him out, what is it's implication on Talos' divinity etc. Essentially, transcending its own crutches or something?
This also hearkens back to the notion of Dwemer building Mecha-Gods (ANUmidium) who instead end up making them disappear (the Mecha Gods were shown to be easily destroyed) because those are only 'powerful' configurations, but not Padomaic nature.
In fact, Magic itself in the setting stems from the Magna-Ge, the light from the Heart of Anuic entities which are uncorrupted - echoing the notion that the Anu assumed they were stronger than they were because in a way that was unstable.
This is also the reason the Daedra embody more 'demonic' concepts - they represent the notion of 'repressed possibility' for the Dreamer which are needed for true 'infinity', while the Aedra embody safe concepts of selection.
This could also be why after the Dragon triumphs over Dagon (who could be said to embody 'indecision' of origin) in Oblivion, that the plot of Skyrim becomes killing the World Eater (and that with the disappearance of the Amulet/Septims, that Talos is being tested). Just as Mankar Cammoran temporarily wore the Amulet of Kings, so did Dagon 'temporarily' walk Mundus as if it was his own realm - only for this Archetype of Troubles to be defeated by the avatar of Akatosh. The Aedra and Mundus are being prepared for 'eternity' and 'immortality' through what is essentially a time-based 'medical cast'
The goal for the Aedra may be for them to be able to defeat their 'perfect Anuic' self with their 'Padomaic immortal' nature, similar to Sheogorath killing Sheogorath-Jyggalag in the Shivering Isles. The 'Jyggalag' in this case is the Thalmor, and the Sheogorath is Talos in Skyrim.