If you count the deaths, and:
Include all Alter Egos (>!Gonta and his Alter Ego count as one!<) plus >!Chiaki as an observer!<
Include >!Daiya!<, >!Tokuichi (Haiji's dad)!< and alternate endings (>!Kyoko and Toko!<)
>!Include all DR2 characters despite them still being alive!<
Exclude >!Taichi (as long as the wiki has his status as "unknown")!<
(For clarification) only count DR1, DR2, UDG, DR3 and DRV3 deaths
Then it turns >!Tsumugi is exactly the 53rd death in the franchise!<
Edit: 2 additional rules
Excluding everything Monokuma-like >!(Monokuma, Usami, Monokubs, Motherkuma etc)!<
Excluding the people who were killed in >!the Tragedy!< and the >!2k Reserve course students!< (despite being valid deaths, excluding them makes >!Tsumugi the 53rd one!<)