If you want the best machamp coutner its good, but in a general sense, yes, a nonshadow ball mewtwo is near worthless. Few match ups through out the entire run of the series other pokemon cannot do better, and it being stuck with the absolute trash move of psychic. Machamp is literally the only current match up it holds supremacy at. (SB mewtwo beats is agaisnt gengar unless its windy) Theres also the threat of eventual community day where mewtwo gets Psystrike, I have a 96% one that I have no idea what to do with, so im in the same boat. I'd just keep it as a trophy, kinda like a shiny
Terrakion was really its only hope for a legendary raid it would be best against, but then meteor mash metagross took that (and with al the weaknesses it had, mewtwo was bound to loose that sooner or later) With Focusblast it can be an ok gym clearer, but meteor mash metagross still beats it there in potion efficiency (and gardevoir beating) and machamp out DPS's it for fastest blissey counter. Ice beam could have had a chance ice Rayquaza was next, but with mamoswine confermed to have the best set it could have gotten, and the three electric types that greatly outpace it with thunderbolt, on top of flamethrower being just bad in general, theres no place for really any mewtwo set aside from psychic, which while its the best psychic attacker, psychic is an irrelevant type agaisnt anything but machamp