Yeah, gimpers have no problem killing Ness.
But it should be kept in mind that Ness can kill rather easily, since he has great kill setups, and that his ease of killing is magnified by the weight of some gimpers, such as MK, Jiggs, and G&W.
DJC and FF fairs into grabs are pretty nice, and can lead to some good stuff. djc fair -> bthrow is legit, and can kill jiggs at 100%. Zapjump fairs are nice for comboing too, and can even lead into a dair depending on DI.
Don't underestimate nair, as it can eat a lot of stuff due to how quick it is. Don't overestimate it either, though. While it is fast, it isn't disjointed, and can't go up against something such as G&W's bair.
As for matchups, I've been playing a burgeoning jiggs, and oddly enough I am finding it rather even. She kills Ness about as easily as Ness kills her, and spacing with fair can help to deal with Jiggs superior mobility. Also, running in with a quick nair to catch jiggs off guard I've found to work sometimes, but not to be something to rely on. Keep in mind, though, that while the jiggs I go up against has used her before, he's doesn't play at a high level, more like mid level or so. So take this for what you will.
MK, on the other hand, I assume would give Ness trouble. MK can easily keep Ness out, can combo him, and can gimp him. While Ness can do the same to MK, he has trouble getting in in order to do the damage in the first place.
However, I firmly believe that Ness' worst matchup would still be G&W. Bucket, bair, and fair really ruin Ness' day. G&W can space ness very well, and can get a full bucket from Ness trying to recover if they can predict the recovery.
Out of curiosity, how would Ness do against Lucas?
Also, alphatron, how do you moonwalk in brawl+?