Why's that? The relationship part's weird, but it makes sense. Pilaf Gang's incompetent to make a wish that backfires, said wish made them younger, then when we get back to Trunks' future, it's believable they'd make a similar wish & then grow up, & that having lived through King Piccolo, the Future Androids, & brief bits with Frieza, Dabura, & Buu before capping off with Goku Black would mean she could easily become an adept fighter, albeit with firearms.
It's fine for Future Trunks & Future Mai since they're both adults, at least. Kid Trunks & De-Aged Mai Who Is Actually Older Than Goku being teased is the only weird part, but I'm sure they can get away from that. Different timelines & what not mean they're not locked-into that relationship.