Can I use a smart sidekick to stain my deck?

Yes. Especially if it's a large a large deck. Cleanup will take you a while so be prepared to do it properly if you want to use it again. Latex stain will obviously be an easier cleanup.

You certainly can use a SMART Sidekick to stain a deck. Being a thinner material, it can be a bit trickier to control the flow of material than it would be a thicker latex paint. This can cause it to be a bit sloppier than you intended. This isn't really a problem if there isn't anything underneath the deck that you're concerned with getting stain on, however you would want to cover or remove anything below the deck boards that you'd prefer not to get stain onto.