I will start!
First I think these low tiers are plain givens as favored
Pichu
Bowser
Zelda
Kirby
Ness
These I think some might argue even but are probably favored
G&W
Roy
DK
These I think could be even but I'd need convincing to be sure
Yoshi
Link
Young Link
Ones I think are losing
Fox
Falco
Jiggs
Shiek
Marth
Peach
Falcon
ICs
Samus
Mario
Dr Mario
Luigi
Ganondorf
Pikachu
Ok, starting with the winning ones, I think this is pretty free,
Pichu
Pichu is just awful and I don't think pichu can approach mewtwo at all, he's got no real punishes, he's too easy for mewtwo to edge gaurd, I think this is really hard for pichu.
Bowser
Bowser is combo food in this mu and it just too big and slow. Shadow ball becomes devastating in this matchup as a zoning tool and bowser won't be able to do anything except koopa claw. Loses neutral and punishes -> loses the matcup.
Zelda
Zelda's toe can be pretty painful, but that's really all she'll have against mewtwo if you stay away from her dsmash.
Camping her with shadow balls and lots of wd back and waiting to punish whiffed toes will make the matchup too hard for zelda. Zelda is so floaty that uthrow and fair kill super early.
Ness
Ness' neutral is the best of these characters so far because of his djc and more speed, but he's still not got much compared to mewtwo. Mewtwo will out combo and out range ness, and won't be killed by the bthrow on medium and large stages. Also mewtwo can teleport out to ness to eat his up b and teleport back, giving ness the same awful recovery as yoshi, no up b. Edge gaurding will be all too easy. Even then ness' light weight and slow falling speed means he'll die of the top very easily.
Kirby
I mean, it's kirby. Any objections? No neutral, no punish, dies early. This one is the most obvious - Kirby sucks.
Ok - hopefully you guys get the idea. You can talk about one matchup, a few, or all of them. If you don't know what you think the matchup is, positive negative or even, don't vote anything yet. but if you're casting a vote make sure to make it clear. If you changed your mind (perfectly ok!) make that clear too!