Technically, you can give it Stealth Rocks but in order to do so you need a smeargle (that has stealth rocks and icicle crash (if you want icicle crash)) and a female Swinub/Piloswine .

Mamoswine can totally learn Stealth Rock.

And oddly enough it may or may not totally be the Pokémon I've been working on breeding with its hidden ability and three different egg moves that I just successfully did... Nope, totally don't want to trust me on that one...

So many ghosts, so many to train other than the current chuckleheads Gengar and Aegislash...

My third team member is finished and I want to give it a go.

Okies!

How good is Yanmega?

Decent. Stealth Rock wrecks it, but Tinted Lens is a great ability and it has the stats to use it.

Gonna go back to hunting my shiny Furfrou now, i'm trying to think of a good nickname for it.

Ice Beam wrecks Garchomp, doesn't stop folks from using it! :lol

So what about Speed Boost Yanmega?

The difference is that you can't use Ice Beam 10 turns before Garchomp comes out and still have it hit.

It's fine, but I think Tinted Lens is the better ability. Yanmega benefits from Speed Boost, but it just never struck me as necessary.