I don't like it either, tbh. Android 21 in FighterZ was unlocked from the start because I preordered it or whatever, and she's on every single one of my teams. Didn't touch the story mode even once and I don't intend to, and that's the only other way to unlock her.
This isn't a super powerful secret weapon you get for doing some quests, this is a mid-high tier character that might suit your playstyle better than most of the cast.
It's not even that I want the character right away, what bugs me is having to play shit tier single player content to unlock the things I want to play the part that actually matters to me. If single player modes in fighting games weren't usually trash, that would be fine.