One of my friends likes to play with Bowser a lot, so I'm fairly familiar with the matchup. Bowser has to be constantly aware of Fair->Grab->Fthrow chaingrab. For this reason, while the fire breath (which probably shouldn't be used much) and bowser's greater ground range in general does keep pika from approaching on the ground, pika is going to be approaching from the air most of the time with the exception of the occasional dsmash when the opportunity comes up. You'll see a lot of fairs and bairs just because it's easy to rack up damage on a big target like bowser with their multi hit attacks.
Its hard not to get thunder-happy when you are playing bowser I've got to admit because if you misjudge the timings, you're almost guaranteed to be hit by a pika who knows what he's doing. While triple thundering isn't a viable edgeguarding strategy against some characters, you can abuse it quite a bit against bowser because he doesn't have much time to be dodging in mid-air before he has to recover. For this reason, sometimes TJolting him can be also serviceable because it can sometimes get him below the vertical range he has for recovery.
Pretty much the only option in the pika matchup is to attempt to shield him coming in and try to get some hits in with the tilts because pika's aerial game>>>>>bowser's aerial game, with the possible exception of bowser's uair, which can be troublesome The thing is though, with bowser, it only takes 70% (and sometimes less) before you can legitimately think of killing pika, while it's going to take at least 150% for pika before he's going to kill you with anything like fsmash and nair unless he's edgeguarding.
So while I'd say Pika has a pretty heavy advantage, Bowser still has a chance if he takes advantage of mistakes because he needs a lot less damage output than pika here, obviously.
