I have played a few mewtwo and I'd say the matchup is even or slightly in corrin's favor I am by no mean a matchup expert but here's what I gathered so far.
Stages:
I think our best stage in the matchup is town & city since he will die super early to Uair and Uthrow. He also can't shark most platforms with Usmash. Battlefield is also a good pick but be prepared to deal with Usmash from below the platorms. Smashville is good too since it's so small and we'll have an easier time pressuring is shadowball charge.
Neutral:
If you play a defensive mewtwo he'll probably want to have shadowball charged before doing anything else, in this case you should backair his shield a few time and try to poke his feet with a tipper IP. Once he has it just shield a lot and try to avoid Side-B on his shield, crossing him up with fullhop Fair is a good mix-up.
If he plays the aggressive game, you have to wall him out with bairs and fairs. The moves you need to respect the most are his Fair, Nair, Usmash and Side-B.
Fair has a huge range and comes out quicker than our Fair and Bair, if the mewtwo is spacing those I'd stay grounded.
Nair is annoying when he lands on you with it but can be challenged with our Nair, retreating fair and pivot Ftilt.
Usmash gets you from under the platforms and slide very far when he does it while running we need to be careful of this option when spacing our aerials.
Side-B is a reflector and a command grab which means it gives him an other option while sharking as well as the option to stun us with our dragon fang shot. If you shield a lot on platform and by the ledge, chances are he will get you with it, this is why I favor jumping away or challenging him with a Dair/IP if he wants to shark.
dealing with Shadowball:
Honestly, like any projectiles, shield is the name of the game. If he juste spam small ones you run/walk and power shield them until you get to him.
If he has a decently charged one, start shielding a lot more and cross him up with Nair or fullhop Fair so he has a hard time catching a landing with it. Patience on the ledge as he can 2 frame you with a ball.
Ledge Game/Off stage:
You do not got off stage against mewtwo! You loose stage control for a small chance at gimping his incredibly fast teleport, you might even get gimped yourself, specially with corrin's... limited recovery.
You should more often than not try to bait aerials through the ledge like Uair or Bair and tipper mewtwo with Fsmash or Side-B.
If he makes it back to the ledge try to punish his get up option.
Recovering:
If you're the one off stage, you're in a dire situation since he has a lot of tools to make sure you don't come back.
If you think he will try to gimp you I suggest you try and challenge him before his move comes out, the only aerials you don't want to trade with are Dair (which gets beat out by draconic ascent unless you recover straight up and he hits you on the head) and nair which will stop you right in your track if it trades.
Welp this is my experience against mewtwo hope this can help somebody!