In general, I'm pretty good at making singleplayer levels, although I've never released any I've made in JJ2. One thing that I think gave me a boost in confidence was when I made 30 levels for a Ludum Dare game I made with my brother.

As for multiplayer... I don't even really play many multiplayer games other than Smash Bros., so I don't really get the mentality enough in most games to make levels for them.