Not completely unviable, but it is far from being a good Pokémon. As a Fighting-type, it already faces competition with Marshadow, Mega Lucario and Mega Blaziken, which all have better abilities, better secondary STAB, Swords Dance (for Lucario and Blaziken), Close Combat and priority (for Lucario and Marshadow). Even as a fast physical Psychic-type attacker it faces competition with Ultra Necrozma, which has Swords Dance, is more unpredictable, can deal with Fairy-types by not going through Ultra Burst, hits harder with its coverage moves, and has access to its extremely powerful Z-Move. Mega Mewtwo X also struggles with Ghost- and Fairy-types that are common on the tier, like Giratina-O, Lunala, Arceus-Fairy and Xerneas.