Because Caulifla, who is clearly a classical Saiyan who values brute strength, takes her in as her own and obviously had reason to believe she has potential?
If she were seriously that weak I can't imagine why she'd even bother.
I dnno. The way I see it, the mirror universes have most things the same except for minor differences (albiet differences that can result in things like Earth being barren of life - though that's more down to a chain of events (like a nuclear war) than any fundamental difference on a foundational level). If you go so far as to give them an entirely separate chain of transformations, I have to start to question why you bothered with Saiyans in particular - why could you not use some other, fresh and new alien race instead, if you're going to change things up that much?
There's also the little thing where Saiyans have by far the largest number of transformations in the entire series, something I still hold against Toriyama and something I really, really wish would change before they give them anything new. Of course, that's not happening now that Goku is getting yet another new form, but dreams are dreams for a reason...
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