I think it should be fine! I'm on accutane now and I use Fakebake flawless and the St Tropez (which I heard is similar to St Moritz). Just make sure you moiturize well. Most people experience very dry skin (I didn't), so you'll need to exfoliate before putting it on.

Doing accutane during the summer can be more dangerous because of the sun and the temptation to get a real tan, but self-tanner is definitely the way to go.

Good luck!