Personally, I used to abuse the crap out of U-smash for damage before Bowser got his Up-throw buff. The thing is, if you catch a fast-faller or even just someone with bad air mobility in an U-smash at low percents you can just keep spamming it until they have enough percent to jump out, where you now get the opportunity to punish landing, or they'll airdodge through it either right into the second hitbox, or right in Klaw range. But whether or not to go for this, usually just depends on what options the character has in that situation.
In my opinion, if you're playing Bowser you should always be looking for early kills. Even if the Corrin had not airdodged, it would have been hard for her to punish the U-smash without getting hit. Most characters in general do not have an answer to a well-timed U-smash except to avoid it completely. Unless you don't hit them at the top of U-smash, counters don't work against it either (with the exception of Bayonetta of course)
You should also make sure to adjust your timing to cover different options, depending on the character. Like a lot of things with Bowser, you need a read. But it's best to make yourself safe from punishment while doing the read. For example, most of the time you'll want to release right when the opponent is in a position in the air where they can hit you, so that Shell Guard eliminates that option. However, if you're playing against a character that has really disjointed aerials or has no landing lag like Sheik, you'll want to release a bit earlier so that you can get that late Shell Guard hitbox protecting you while at the same time having a bit more frames in order to act in the case that your opponent has made all of the right decisions to not get hit and try to punish you later. Remember, we pick this option hoping for the best, yet preparing for the worst. Best case scenario, they eat an U-smash for 20% and/or death. Worst case scenario, you get stuck in a situation in which you need to pick an option in order to not get punished. Usually, things will just get reset to neutral.
The main thing is that you have to consider what space your opponent can occupy in the 1 second it takes to throw U-smash. It's better the closer they are to the ledge, and they have nowhere to land. A secondary use of this move is simply the fear of it to command space. But that won't happen if you just throw it out in neutral so don't do that.
