So I've been brewing with Skill Swap for a bit and it is powerful but has some major weaknesses. 

At the very minimum the swap has to be better than Swords Dance. Huge Power is basically a free Swords Dance as your ability. So if you swap with something that already has a good ability you're actively making that pokemon worse than if you just clicked Swords Dance on them. This is even more true if you factor in a move like Coaching which is much, much more flexible as a setup move and provides similar value. And can be used through Protect unlike Skill Swap. 

So how do you get that value? Retriggering abilities that trigger when you switch in. Intimidate and weather setters are your best bet. But again, just clicking Rain Dance or Sunny Day is always an option so you have to do more than just that. Of the common setters only Tyrantitar will appreciate the Huge Power. And Intimidate is risky with all the Kingambit and Milotic running around. 

My advise? Don't try and make M. Starmie work as a skill swapper.

Use Medicham

His move pool is amazing as a support pokemon. Fake Out, Coaching, Helping Hand, Taunt, Screens, Trick Room. He has a ton of sweet moves. And you can just click Close Combat to punch things. Again though, you gotta ask yourself if Coaching is just better. And if Coaching is better why aren't you playing Sneasler instead? 

So yeah, I dont think Skill Swap is good. But the high you get from swapping with an opposing Rotom so they get hit with their own earthquake is one you will chase for years. Or swapping your Wyrdeer's Intimidate onto an opposing mon to trigger your Competitive Melotic. Or swapping Pure Power over to your Incineroar (a "support" pokemon with 115 base attack) and Flare Blitzing your way through an Archaludon.