mount covers

Those aren't covers, though. They're special limited time event mounts that have their own unique level, speed, and so on. Mount covers have never been done before (though clearly mounts were designed with this thought in mind) so we don't really know how it will work. Will they have separate level, and tiredness? Will they have different speeds? There's no telling. Calling it a "cover" suggests that it wouldn't alter the stats in any way, but you never know.

If Nexon keep to their word; NX items will not change your stats in-game.

(x2 exp/drop =/= stats)

So, Mount Covers should not affect speed/jump/etc.

For the last time, we will be the first MapleStory to have mount covers, don't get too excited, they might be shyte.