He actually had very little start up and ending lag for his moves. He only had 2 real kill moves that were used frequently, his approach was good with his fair, but required a little finnese to get perfect. Pretty much every single part of Ness was more than good, but he was such a floaty little ***** that he got comboed incredibly easy by the likes of Fox, Falco, Marth, and Shiek, and his combos against them were weak at best. Marth's range is incredibly hard to overcome, and even if you do, he has combo which are just beastly against Ness. Fox and Falco just have uairs and shines and dairs, and such, which used against Ness, can get about 40-80% damage on him with minimal chance of escape through DI. Sheik has priority out the ***, so it's difficult to approach her with Ness, and she can chaingrab him at early percents, and then combo her killing aerials after grabs in the later percents.
And of course the only combo Ness has his uthrow uthrow uair uthrow uair uair uair uair, against Fox and Falco, which is, by itself, rather difficult to master.
This coupled with a few other factors are what made Ness bottom tier.