yay discussion and... respects :D
Well a -1 is a slight disadvantage, pretty close to even. This also means that it somewhat depends on ruleset.
For example I believe Wario is an easier matchup playing at 3 stocks than at 2 (just an example).
So about Diddy and Sonic... I believe we are Diddy's worst matchup (alongside Olimar) after the ditto. Maybe not but we def are no easy matchup for him. That's because we have hella good options against Diddy. Even though, the fact that diddy is super broken is real.
Sonic is another super good character we should shut down, but just like diddy he's overpowered atm. This means he gets more reward than he should out of his options and stuff like that, which helps him have better odds in this matchups than would if he was balanced.
Anyways I believe both matchups are evenish, although diddy is still hella hard and it's only close to even in our theoretycal minds where we have achieved perfect play (someday...).
Sheik I believe is close to unwinnable at highest level. This means Mr R, Nietono and the likes. He just like shuts us down. With customs things may change a bit, Beat is definitively an improve. At just high level and below it is winnable.
Greninja ****ty grab, worse dash attack than fox and approach options limited to aerials give him a harder time. And he can't punish long range as fox can. He has no good kill options in neutral game besides high risk side B. He really troubles against pellets imo. Fox doesn't as much.
Ok fox got on us -> nair is great and all (just like usual), lasers can punish our camp sometimes, enough to make us move around, if he can get a jab in at mid % and is proficient with the jab chains it's gonna hurt, dsmash can wreck rush recovery if he's good enough with the frame vulnerability, combos us for days, overall good mobility (he's kinda like poor man's falcon). Overall he's very hard to hit with anything but you either play it super duper defensive or try and get some early kill because of him being light. I believe its a slight advantage for Fox. His main problem is that he doesn't really have reliable kill moves unless he's super good at getting the dsmash on ledge frame vulnerability, and it can lead us to getting a good kill with rage effect.
Huhhh i don't really want to write too long about each char so i'll leave it at it.
Mostly i believe chars with long lasting hitboxes in aerials and great air speed are hard for MM. of course they gotta have more than that. Hard combos, hard to gimp, strong edgeguard game or fast frame data are also good.
So i think it's bad when they can approach us and combo hard us since we are heavy and fast faller (which requires a way to get through pellets) and they are very hard to get hit with a kill move (recoveries like ZSS' or Sheik are nearly ingimpable, superb frame data makes us not be able to punish stuff with ko moves, leaving only reads for kills). There are more factors but those two I believe are the most important.
Choco (zss) destroys japanese megamen btw. A single example isn't worth anything but just saying. I have played the matchup against Salem and Leon.
Anyways most -1 may be even and i'm just being pessimist. Or not. Mario and Pika are ones I'm edgy but I firmly think Yoshi and ZSS have the upper hand in the MU.