My reasons:
Mustard's design is better; I know it's simple, TOO SIMPLE, but comparatively it's better than Ojiro's. There's no point for either of them but I want to make it clear that Mustard design is better in its simplicity.
The hair: LISTEN, I KNOW MUSTARD WEARS A HELMET BUT IN ONE SCENE HE DOESN'T HAVE IT AND HE HAS BETTER HAIR THAN OJIRO. Point for Mustard (Ojiro, slime your barber plz.)
The name: The name "Mustard" refers to mustard gas, a highly toxic oily liquid used in World War I, which is a good reference. Tailman is a very lazy and uncreative name (what else can you expect from the potential man?) Point for Mustard.
Mustard's quirk is better; Let's see... It's similar to Midnight's BUT BETTER because he can detect enemies as long as they're inside the gas, and it doesn't have a fixed creation limit. The only disadvantage is that he is physically weak and his gas affects him too, BUT with a gas mask and a revolver, everything is solved.
Ojiro's quirk is... IT'S A TAIL, WHAT DO YOU EXPECT? The only good thing about this quirk is its user because Ojiro is very capable of using it and knows very well how to do it, I have nothing more to say; I find it mediocre in the sense that if they literally cut it off, you'd be a normal person. Point for Ojiro and Mustard, both quirks are good in their own way, one better than the other to be honest.
Story: Let's see... I don't have much to tell you about Mustard because he literally only appeared in one chapter, BUT what he said suggests that he's a guy with a clear hatred and jealousy for UA and its students WHY? We don't know, but it could be that he failed the physical exam because of his quirk, which isn't designed to work against robots (besides, i don't believe they allowed him to use a gun). He talks about UA students as people spoiled by the world but unfortunately simple, it makes me think he says that because of their quirks, because they have something more physical and aggressive than tactical and passive like him. He's scrawny, so I don't think he could have destroyed the robots with his bare hands And most likely, if he managed to stop a robot by sheer miracle, another student would steal the kill. Which gives us a character who is more or less interesting because of his motivation driven by jealousy (And also because of Stain's ideals).
But Ojiro... DOES ANYONE GENUINELY CARE? He's not interesting, and I don't think he had any dialogue or anything that's even remotely important. HE'S THE GUY IN THE TAIL, what did you expect? HE'S ALWAYS IN THE BACKGROUND. A well-deserved point for Mustard for appearing in only one episode and having a better storyline than Ojiro.
In short, Mustard: 4 and Ojiro: 1
Mustard, my goat, In another universe, he was a redeemed villain who, after a long time, entered UA and became a pro hero.