Off the top of my head,
Yoshi
Maybe zard
Fox
I actually suspect he loses to link
Rob
Roy
D3
Pikachu I've been told
Ganon imo
Likely Diddy but I'm not sure
Cf definitely
Falco probably - I think b reverse lasers change this matchup a LOT more than falcos nerfs from ntsc
I don't have a cast roster in front of me atm though. The trouble with Marth isn't his overall spread, it's the fact that the MUs where he wins, he tends to completely demolish them.
I see your points, but for the same reason you say Bowser gets bodied by so many characters, I think if Marth plays the matchups correctly he shouldn't lose most of these. Here's my opinions
Yoshi- Marth beats yoshi for sure. He may have a hard time killing Yoshi if he doesn't get a spike or nice tipper f smash, but it doesn't matter because if the marth knows what he's doing he will just keep winning neutral/keep edgeguarding til yoshi finally dies. Marth's disjoint is just insanely good and safe against Yoshi. doesn't have the range and/or speed to deal with Marth's neutral assuming 2 equally skilled players are playing each other.
Maybe Zard- I think Marth's ability to keep opponent's above him play a huge role in making this matchup good for Marth. I know this because I play DK who also has that same strength, and this + edgeguarding are all you really need to beat Zard. Marth does both of those very well. Also, he has a great shield grab which could be used as a last resort if Marth can't stay below Zard and he tries doing a fast fall aerial to the side to get to the ground. Just run up + shield grab, or simply have a good DD grab game to punish his landing, and it's hella hard for Zard to land safely.
Fox- This was about even in melee and Marth is essentially buffed in PM due to his less lag on many aerials and stuff, and obviously fox is nerfed, so i would say this is even at worst for Marth.
Link- This could be a secret losing matchup Marth has because it was Link's ONE good high tier matchup in melee, and Link is obviously buffed from melee. Aerial mobility is a key trait against Link and Marth is mainly a grounded character. Therefore, he has to resort to amazing powershield game unless the Link simply gets outplayed and Marth consistently finds his way in.
Rob- Marth beats ROB cause grabs and edgeguards lol. Marth has a very easy time grabbing ROB because of his maneuverability, and it's very easy to get ROB offstage from a grab. I would say DK has an easier time dodging dairs than ROB does. That's how free it is for Marth to edgeguard ROB...
Roy- Can't really say much hear, don't have enough experience. However, One thing I do know is that if Roy is above Marth he's pretty much dead because being a semi fast faller he has to DI offstage in order to not get combo'd forever, and from there all he really has is his invincible ledgedash lol
D3 - Ever see Sora v.s. Ripple? Sora had like zero DDD experience compared to Ripple who plays Dart's Marth all the time. Seemed like if Sora had more experience he would have had an easy time winning neutral over and over again due to his DD grab game, and easily transitioning into dair edgeguards. Also, Ripple is really good anyway so it's not like Sora was super outplaying Ripple or anything lol. Sora actually took a set off Ripple despite the matchup inexperience. It looked pretty stressful for Ripple. Sora played super safe and did a lot of work when he got in. Keep in mind he did all this while figuring out Ripple as well as DDD. I would like to see how well Sora would do in that matchup if he had Dart's MU experience
Pikachu - I might agree with this actually lol. Idk how it is in melee, but it's prob safe to say that if Pika does decent vs Marth in melee then it's even better for him in PM
Ganon- I agree with this too. Marth is one of Ganon's best high tier matchups in melee (arguably even) and Ganon is way better in PM than melee. Especially his movement, which is something that Marth easily abused in Melee.
Diddy- I think Marth wins because I've watched Sora body Diddy's back in 3.02 Haha i have a biased perspective admittedly, but still. I think Ground mobility is huge v.s. Diddy because Marth can shield a banana, then quickly get in while running under the banana as it's in the air (assuming Diddy throws the banana from long range). Jumping is a big commitment v.s. Diddy (not gonna explain why) so ground mobility is huge. Also, Marth easily converts grabs into edgeguards Diddy, and he's is hands down one of the best characters in the game at edgeguarding Diddy.
Falcon - Pretty much even in melee, pretty much even in PM as well. If anything, it's better for Marth because he can wall Falcon out a little better with less laggy aerial pressure than in melee.
Falco - I actually agree with b-reverse lasers being huge in this matchup. It makes his neutral much less predictable. Falco may win slightly like 55-45. Prob even at Worst for Falco
Btw, can't forget Sheik. Sheik does pretty damn good v.s. Marth