It can't be ok if you're not a Yoshi to begin with. At least the NES game made some kind of sense (the king was a Pirahna Plant, wasn't he?) Come to think of it, the SNES game still made sense, because Yoshies can eat Pirahna Plants, and what better way to doom a world made of pipes than to turn their leader into a ravenous dinosaur (unless the king still has control over his mind, which means he still controls what he eats..)