Against Marth, you were definitely too aerial. Marth's gonna fair you a lot in this MU, so you should stay grounded, at least in the beginning. I typically start grounded against Marths (upangled ftilt, mostly) and see if they change their tactics at all. Usually they'll mix things up a bit, so then I'll be a bit more aerial.
Watch your use of SH fair--it has enough lag on the ground to be punished.
All your dairs against Marth were poorly planned. You rarely, if ever, use dair from really high up, as it's easy for your enemy to SDI out of it and punish you. Dair's best as a mixup for shieldpoking. People don't expect dair too often from Bowser, and ftilt/fsmash can be used as a followup since dair pushes them back a tad.
Overall you started off too aggressive, got in a hole, and then played Marth's game for the duration of the match. Just settle down and outspace him--Bowser certainly has the tools.
Against Diddy, too aerial again.
You PS'd a bit before the 2:00 mark and didn't Fortress. Both of you just stood there.
Basically this is my theory on Klaw vs. fair as a means of aerial spacing: I prefer Klawing over fairing when no one's near me because a good player will think "Dang, this Bowser's throwing out Klaw a lot. I better shield less." THAT's when I start using fair more. Your mentality seems to be the opposite.
Against Kirby, good call Klawing his Stone first time around. Second time, though, he jumped out of it because he predicted Klaw--if you spot Kirbies doing this (which most do anyways so they don't get punished), just run in and upsmash Kirby. Works really well.
Random tidbit: If Kirby's dash attacking you a lot, just pivot grab him.
Why do you like to SH>airdodge as Bowser when no one's near you? I don't like showing my opponent SH>airdodge at all--I don't want them to recognize the timing behind it or even to think I'll attempt that at all as Bowser. You SH>airdodged a handful of times when Kirby was offstage only to SH>airdodge>uptilt when Kirby returned onstage. He predicted that a mile away because you were being flashy. Bowser's bad. Really bad. It helps us because people underestimate the character. Keep your trump cards hidden so your opponent doesn't expect it.
Around the 1:20ish mark or so, you power shielded Kirby's attack and didn't capitalize on it. A PS in close quarters like that is a free Fortress OoS.
You need to decide how you want to incorporate Koopa Hopping into your Bowser game. You like doing run>jump back>aerial Klaw (which I do a lot and dig), but I rarely see you perform multiple Koopa Hops in a row. It's really hard for me to recommend how much you should Koopa Hop, as all of us use it really differently. It would definitely help give you more mixups when approaching/camping, though. Right now your Klaw game is very basic. Koopa Hopping more also (moreso against characters without a projectile) makes your opponent respect your space a bit more.
You were way too aggressive against Kirby on RC. When I play on RC, I almost NEVER approach on the middle part of the stage, especially if I'm up against a character with multiple jumps like MK, Kirby, etc. You started off in a hole and seemed to want to bring the match back quick. Just remind yourself it's Kirby. He's ridiculously light compared to you and is rather straightforward. Don't pay attention to the percent difference. Let him come to you more, especially in that middle part.
I hope this helps! If you need anything clarified or have any questions, don't hesitate to ask.
On a side note, I saved a ton of matches from the tourney I was at like 3 weeks ago, but Mike hasn't had his Wii and his laptop together since then, so it's taking a while to upload. I'm also still trying to get my MM with Razer up from like 3 months ago. XD
I'll have a good bit of videos recorded at Genesis 2. I've got a lot of MMs lined up--I'll get as many of those saved as I can along with pools stuff. Biggest problem for me will be if my arms can hold up for that long. I've been stretching and conditioning them every day, so we'll see.