I would recommend not using the dair. It's laggy (which, I guess most of Bowser's moves are) but more than that, it doesn't clear your opponent out and it doesn't combo into anything, really. Which means your opponent is going to be right next to you while you're lagging, which is bad. Try using the nair instead.
You're doublejumping a lot... which probably isn't a good thing. For one, you're opening yourself up to a lot of juggles, since Bowser doesn't have any good ways to get down and you're a pretty big target for juggling. Secondly, if you get knocked off the stage, you want to have your options open, and to be able to get that bit of vertical distance, little as it may be. In any case, Bowser's doublejump probably won't fake anyone out. If you need to break out of tumble, wiggle out. If you think your opponent will shield, Koopa Klaw. Doublejumping is rarely your best option.
Once you land a hit on a character like Young Link (this applies to all characters, but especially for a projectile heavy character like Young Link) you need to follow him closely. You don't want to have to wade through all the projectiles again to get him, right? Don't be afraid of close quarter combat, you have the Whirling Fortress!
Also, play four stock and don't miss L-cancels.
I would almost recommend not spotdodging. Bowser's spotdodge is really really punishable, especially when you're not moving. Try rolling or jumping instead... or the ever popular Fortress out of shield!
Grab more. Grabs are fairly safe, reasonably fast, unshieldable and most importantly, give you total control of the match, at least for that moment. You can dthrow for the techchase and corner him in a bad position, uthrow to try for a combo or use the fthrow or bthrow to set up an edgeguard. Whatever you like.
When your opponent is standing right up next to the ledge, try ledgehop Klaw. A lot of people will see the ledgehop and try to shieldgrab, and you can grab and throw them off for the edgeguard. Or you could just grab and throw them off even if they don't shield. It's good stuff. And just in general, make sure you ledgestall before you ledgehop and just ledgestall in general. Nothing sucks more than getting hit off the ledge.
Shffl'd uairs on the approach really aren't a good idea. If you want to pop them up, grab and uthrow instead, or Klaw and fthrow.
Fsmashes are cool and funny to hit with, but don't forget about the ftilt.
