Yeah, that's just the deal with actors. Cobra Kai is a role to them, not necessarily a lifestyle, so whether or not they really embrace martials arts and training isn't relevant. Mary Mouser is almost THIRTY, maybe she is having kids between seasons? And while CK is the only thing she seems to have going on, I doubt hard core MMA training is a part of her contract or a particular interest to her. She has no sex scenes or much that really requires her to be in good shape, the wardrobe team does an ok job.
Tanner Buchanan (Robby) and especially Peyton List (Tory) seem more focused on physique and athletics but even they look more "GQ model" than a fighter.