It's not the worst. Compared to other dragons it lacks in speed. Its attacking stats are good but not great and they're a bit unfocused. It lacks strong fighting STAB, its strongest physical one is Sky Uppercut which isn't too bad but it's a lot weaker than the Close Combat a lot of other fighting types get, and on the special side it only gets Focus Blast which suffers from horrible accuracy. It does get a lot of boosting moves in Work Up, Belly Drum, Bulk Up, Dragon Dance, Autotomise, Rock Polish, and Swords Dance, of which Dragon Dance is probably the most viable, although Salamence is probably a lot better at that with a much larger attack stat, easier boosting thanks to Intimidate, and better speed. Kommo-o is by no means a bad pokémon, but it suffers a bit from "Jack of all trades, master of none" syndrome. I think it'll probably gain a lot of viability with the new Z-Move and possibly move tutors. If Superpower gets distributed again I think Kommo-o will get it, and it might be one of the best users of it on a mixed set with Clanging Scales, Flamethrower, and Poison Jab.
Whether the new Z-Move will make it overpowered? I doubt it. It'd definitely make it a force to be reckoned with, but I'm not sure on overpowered. It's a Dragon move so fairy types are immune, and with the Tapus being so rampant as they are it might struggle to find an opening to actually use it, because if you use it while Tapu Fini switches in you've just wasted your only Z-Move for the battle.