MU is 70:30 in Falcon's favor or worse imo.
Mewtwo has to play extremely carefully in this MU. Any whiffed moves spell trouble for the M2 player, since Falcon's combo game and KO abilities are significantly stronger than Mewtwo's. Every dash grab Falcon lands equates to 1/3 of Mewtwo's stock being lost since Falcon due to his strong combos out of it (he can either choose to juggle us w/ U-Air or hit us w/ knee, both of which can deal massive damage to M2). Falcon also edge guards us hard since his back air and knee have lingering frames, allowing them to intercept Mewtwo's recovery.
Mewtwo loses the neutral badly, which is bad since his disadvantageous state is awful in this MU. Falcon's frame 2 jab allows him to beat Mewtwo out in cqc, and his vastly superior dash grab and dash attack give him a much better mid range game. Mewtwo will hardly get a chance to charge shadow ball since falcon is so fast and can easily punish M2 for doing so. Tiny shadow balls also don't have much use in this MU since Falcon's Dash attack cancels them out. They still are good for trapping landings though.
Mewtwo should watch out for Falcon's F-Smash, which is basically safe on his shield due to his bottom tier traction and can catch his landings. Don't ever try punishing Falcon for charging one since Mewtwo's range and frame data are both very poor, meaning that you'll get hit by the F-Smash in most situations. Rage Raptor boost, Up Air, and Bair are also very scary for Mewtwo since they can all KO him very early.
Mewtwo's advantageous state against Falcon is decent, but tame compared to other characters like Doctor Mario and sheik. Our lack of grab combos means that we can only get a positional advantage against Falcon instead of comboing him to death like other characters can. D-tilt to fair is the only notable combo I can think of that works well on Falcon.
We edge guard Falcon pretty efficiently, but again, our edge guards are very tame in comparison to other characters. Bair is very efficient for edge guarding. Fair and Nair also work, but you need to space them precisely since falcon can punish whiffed nairs and fairs with knee.
Sorry, but if you're getting edgeguarded by Falcon offstage then you need to work on your recovery mixups. Mewtwo can easily stall out and counter with bair or teleport past him once he jumps off to get you. He can't go far to get you. I can't stress this enough: Mewtwo only really gets gimped if he makes a mistake.
If Falcon is in a position where he can get gimped by nair, then chances are he can't throw out an aerial without risk of dying due to how fast he falls, especially with the laggy knee. Even so, you can just throw a weak shadow ball to provoke a reaction first to avoid such a thing.
Mewtwo doesn't lose neutral that badly with the low-endlag dsmash which at least keeps Falcon from running in willy-nilly against Mewtwo and shadow balls, even the little ones, slow him down considerably, and if you think Mewtwo has bad range maybe you need to stop trying to grab so much. Falcon also doesn't really have many safe approaches either, with dash grab and bairs being the most reliable ones as dash attack is punishable, which simplifies your defence as you know what to look out for.
Try a disable as a whiff punish versus fsmash, or just throw a shadow ball if he's charging it.
Any positional advantage you get against Falcon with grabs is very rewarding, whether with fthrow (easily gets you a decent ball), uthrow (since Falcon resists vertical KOs keeping it fresh is less important and Falcon's landing options are bupkiss) or dthrow (chasing his landing if he jumps after is easy, or you can plant him on a platform), because he is even worse at landing than Mewtwo. At least he has various mobility mixups and airspeed to work with, plus nair is actually somewhat decent against a grounded Falcon as he lacks a quick anti-air. Still, if it's guaranteed strings you want, I can easily convert nearly 50% on a low percent Falcon off of dtilt with fair->uair->usmash + further juggles. It may not be as easy as Falcon juggles but it's just as rewarding.
All in all this matchup just isn't as difficult as someone like Fox, who not only has more varied and safe approaches but also can counter shadow ball. Falcon may be able to get in and cause significant damage fairly easily but a smart Mewtwo can minimize those openings. No worse than 60-40 Falcon's favour. With practice he becomes significantly easier.
