I wouldn't singlespeed a mountain bike. They are heavier than road bikes and you will need to put more effort to accelerate and keep it moving especially on one gear. A good compromise is to ditch the front derailleur and run a 1 x N drivetrain. You'll still have a wide variety to gears and you don't have to deal with the slow and often imprecise indexing of the front derailleur plus if you run the chain on the middle chainring, you don't have to worry about cross chaining your gears.

Singlespeed mountain bikes like mountain bikes in general is more suitable for mountains. Who'd have guess it?