He's still good, just needs theory put into practice. None of his options are particularly dominant taken individually, his movement is rather demanding in terms of precision, and his mental game is also rather demanding in its own way because of how he can juggle different options and strategies depending on the situation. Bringing it all together is tricky, especially against a character like Falcon that can smother partially-developed gameplans with his DD. It's doable, but Falcon players arguably have a better understanding of their own play & how to execute it than Mewtwo players. That's no small advantage. Give it time.