If you're defending Mario's D-air then I can only come to two conclusions: A) You're a Mario main and you're crying someone pointed out your BS, or B) You've never played against a Mario who understood how broken the move is.
A) No, not a mario main. I mean, you see the icons below my avatar, right? I played him in smash64 but thats it.
B) FG is littered with marios spamming dair. It's a pretty common move.
The way I see it, there are only 3 possible explanations for the trouble you're having with these characters.
1) You're a scrub
2) You're a scrub
3) No seriously, you're a scrub and you need to git gud
So let me get this straight. You're outright ADMITTING that Mario is unpunisable when against Bowser?
I didnt say that, now did I? I said, clear as day, that Bowser may be too slow to punish dair. Other characters can do it, especially if they space themselves correctly and dodge roll. It sounds like your problem is trying to shield everything and then get an instant punish, but you clearly dont have a handle on the concepts of frame data and recovery frames.
Bowser can punish dair, though. Here are two ways you can do it:
1) Upsmash. It has super armor, so you'll power right through it.
2) UpB out of shield. Time it right, and press upB (without letting go of the shield button) and you can punish it.
Now, are you gonna stop being a scrub-a-dub-dub and git gud, or are you just another lost cause?