A skin won't cause the Spark to overheat. Just make sure the air vents aren't blocked at all and you'll be fine.
I'd avoid a black skin though. If that sat out in the sun it could possibly get too hot. At least that's one theory why DJI doesn't offer the Spark with a black cover.
I avoided the temptation to put on a skin. Once the drone is above eye level they all look the same anyway. If you don't like the color replacement covers are only $10 on eBay or Amazon.
Enjoy