The main thing I noticed was the complete lack of jabs/fortress. Those are both really good moves, and the former can cancel into a klaw, fortress, grab, and even a ftilt or dtilt if you catch your opponent off guard. Upb, like Liquid Gen said, is Bowser's best out of shield option and is really the one move that makes Bowser such a defensive fortress (get it).
I liked that you were using aerials (sh fair in particular), but you should really cut down on his other aerials. You should only really be using bair from a full hop or while edgeguarding, because otherwise you will get punished from the ridiculously long ending lag. Nair is a pretty situational move that shouldn't be used as much as you used it. I liked the upair kill in the first match though
Actually, you should just stop being in the air so much in general. Klawhopping does look cool, yes, but you also don't want to use it too much because Bowser has a huge weak spot below him. Against ROB you want to stay grounded and try to force him into the air (ROB also has a big weak spot below him).
As for killing ROB, you want to try to kill him vertically, both because of his weak spot and because his recovery makes edgeguarding very hard. Upsmash, uptilt and upair are all excellent vertical kill moves ("vertikill moves" if you will
). Again, I liked the upair kill in the first match, but I didn't see any uptilts or upsmashes. These are especially good at killing on stages like Battlefield where you can attack from beneath the platforms.
Stop air dodging when you are off the stage and close to the edge
You died twice doing that.
You got punished for using fire breath a lot. Either stop using it so much or try to space it better.
I also noticed you got punished for some seemingly random fsmashes. Fsmash is best used to punish poor spacing by your opponent. For example, ROB's sh double fairs can be punished by fsmash as long as you time it right.
If you do happen to get above your opponent, instead of trying to air dodge back to the ground all the time or using an aerial (and getting punished by the lag), try DIing over to the edge and using a Bowser Bomb. I saw you use it once in the second match, but it's overall a pretty reliable way to get back on even ground with your opponent. With that being said, try to be a bit more creative and mix it up once you are on the edge. Bowser's ledge attack isn't nearly as good as his <100% ledge attack in melee, and it's a lot easier to punish.
Your Bowser isn't total trash at all
You just need to stop using your laggy attacks (bair, nair, etc) and use more of your quick attacks (jab, fortress, uptilt). It will make your Bowser overall a lot faster and harder to punish.