I don't think
Mewtwo-GX is bad, but it isn't especially good, either.
is it cuz of the attack not being colorless?
If you're comparing it to the original
Mewtwo-EX, besides having an attack for [CC] in a format where many/most decks were running
Double Colorless Energy or had compatible Energy acceleration, that was indeed a significant part of its success. X-Ball did damage based on Energy attached to
both Active Pokémon, allowing it to punish small things with Energy, big things with a
lot of Energy, or almost anything Psychic Weak. Another was
timing:
Mewtwo-EX (NXD) released in the first wave of Pokémon-EX and was far and away the best of them. It got to the point where about the only counter for
Mewtwo-EX was your
own copy.
Those don't apply to
Mewtwo-GX.
Maybe you could consider running
one copy in something like a
Metagross-GX deck;
Metagross-GX has an Ability that attaches a basic
Metal Energy or
Psychic Energy card from your discard pile to your Active. This would allow a single
Mewtwo-GX to slip into the deck to counter certain walls. The
catch is that we're still talking about its GX-attack, which is a one time deal
and the deck has at least one, probably two other GX-attacks it would rather use.
Mewtwo-GX also suffers from its Weakness; while not every deck is packing a Psychic-Type, enough are that 190 HP becomes an easy OHKO. I thought about maybe trying
Mewtwo-GX in Expanded, where
Dimension Valley plus
Choice Band allows it to do 90 for [P] while healing 30 from itself, at least against Pokémon-EX/GX.
Muscle Band can be swapped in against regular Pokémon for 80 damage/30 healing, or maybe
Fighting Fury Belt if for only 70 damage and 30 healing
but bumping
Mewtwo-GX up to 230 HP. Keeping it attached, though, probably not likely, and needing [PC] is why I didn't suggest the combo for Standard.