People really like this argument, and it's such a bad-faith one.
Umm, yes... an alien dog that can fly and somehow understand complex human instructions, and is also somehow super long living is far far less believable than an alien monster coming to Earth.
Degree is a thing. Maybe I can suspend my disbelief about an alien monster being brought here by a malevolent force, but the more you force me to do it.... by doing things like having flying dogs or regular people not dying, etc... you make it harder and harder to do so. And thus more difficult it is for the stakes to feel real, and for the suspense to not feel forced. What you are essentially saying is, "since we have a character that can fly, that means that physics or likeliness no longer have to be a concern." And I'm sorry, that's nonsense.
Krypto is a dumb idea made up for ten year olds. Yes... Superman is for 10 year olds, but he's for 16 year olds too, 18 year olds too, 30 year olds too. And grown up audiences have to like this movie. The only thing he does is lessen the stakes and make the whole thing feel like less of a threat and more like a joyride.