corviknights got mirror armor
That isn't Corviknight's best ability. Pressure is generally better for being able to PP stall Stealth Rock and Spikes with Defog, something that isn't possible with Mirror Armor.
Now, onto the question itself. There is no true answer to Skarmory vs Corviknight in general, as both perform different roles. Both have got a lot going for them. Corviknight's Pressure, good matchup against most hazard setters, typing, and the ability to fit Defog on its sets easily make it an excellent Defogger, considered to be the best one by many players. At the time when this question was posted, Skarm's presence in the metagame was small, with it generally being outclassed by Ferrothorn as a bulky Spikes setter, and by Ash-Greninja if you're playing National Dex for its power and Spikes wearing down its checks. It had access to Defog but using Defog would make Spikes significantly worse, which is why it can't really fit the move on its sets. However, recently, Skarmory has started to pop up as one of the most reliable Spikes setters. Unlike Ferrothorn and Ash-Greninja, Skarmory has reliable recovery, letting it stay healthy throughout a game. Its base 140 Defense stat also makes it a good tank, using Spikes consistently throughout a game, Roosting off damage, and spreading Toxic / boosting its physical bulk with Iron Defense, which also powers up its beefy Body Press. Unlike Corviknight, Skarm also has Toxic.
Corviknight is a reliable Defogger and Skarmory is a great Spiker. The answer to your question will depend on what your team needs, what it has and doesn't have.
