How many of you *actually* play Marth? Jesus you people are real depressed about his MU's. His MU's are so underrated, it's no wonder people take "Marth is low tier" as Sunday Sermon!
Get better managing bananas, in all aspects. You can:
Swat them
Pick them up with wavedashing/wavelanding
Accept banana hits in the air to avoid the trip + you tech (you can actually recover faster by doing this if you are low enough to the ground, than if you tripped and teched normally iirc)
Glide toss them back or AGT them in various ways
Z Drop them (try jumping over Diddy, and Z dropping + threatening to footstool if they shield camp)
Hold onto one, so that you stop the apparently "devilish" setup of 1 Banana on the floor + 1 Banana in hand
If Diddy has a Banana on the floor, why does this stop you? "Oh sheeeit, I can'd DD in that spot anymore!" Ok then, analyze that spot. How far away is that Banana placed? If it's too far from Diddy, what the hell stops you from picking that bad boy up? Fear? Ignorance? Go pick that thing up, littering is bad. If you're spaced so far away, that Diddy can't punish you for quickly wavedashing to pick it up, then frankly you probably don't *need* to worry about your DD at that range. You're falsely being "limited" at those ranges because of the lack of executing the remedy.
If it's right in front of Diddy, and he's shielding/camping/doing whatever, then get up close with him too. Empty jump near the banana: if he goes for an attack you can swat at him. If he's shield camping, baiting a messed up approach where you might trip on the banana already on the floor, you can opt to go through him, over him, waveland backwards on top of the banana to catch it, etc
Someone like Marth can also extend attacks past the Banana spaced on the floor. You can still tilt and Fair around that area.
If you play a stronger projectile character, you will laugh at Bananas on the floor because SPOILER ALERT I'M MARIO/LINK/JESUS BABY AND I CAME TO BALL
I think Diddy's neutral is decent, but it's much more based on getting options for dealing with mistakes, rather than forcing mistakes. You "force" mistakes by a person being impatient or ignorant of how they can handle his projectiles or campy setups. It's more akin to "Oh hey, I can bait certain things with a Banana in hand and get decent punishes that I might have missed out on otherwise", instead of "My dash speed/grab range/shield pressure/extreme tech chasing lead my opponent to have no options". He's kind of overrated, and on a more minor note, people tend to assume that he kills somehow. Diddy is living that struggle on trying to find overpowered kill options. He really is. People only reliably list Fair in that department (the issues with Dsmash and Fsmash for being staple are on the wall), and you better get your ass way offstage sometimes to make that dream come true. Maybe find the cheese in your soul and try to Upb kill people.
As for other Marth MU's, it's true that he probably loses to Zard, but I dunno if I agree with a lot of Brawl characters that get tossed around as "Oh yeah he totally wins vs Marth". I think people just assume that Brawl characters will get super duper beefed up in Metagame knowledge, and suddenly MK is #1 instead of Fox and Marth can't beat Ike or TL. So on and so forth. Brawl characters get a lot of free passes on potential or future advancement, and it really shows in how people rate Marth's MU's vs Brawl characters.