Looks like you've got the basic idea down. Anyway, Peach is tough, even for Ness. Peach mains will often times try to "glide toss" their Turnips at you. It makes good use of attacking and spacing for them. Ness can't really glide toss. His is probably one of the worst ones. Another thing to look out when you fight Peach is that she'll Float > dair. Never spot dodge when she's floating. You'll get beat really bad. Usually when Peach floats, she'll try to KO you with her fair or nair.
When Peach is grounded, she'll probably try to use her u-tilt. Oddly enough it's got a bit of range on it and prepares for a good combo against you. With that said, she'll probably try to slap you first and then do an u-tilt. It's not hard to get away from her u-tilt, though. If you do get hit, just get away from her or you'll just get hit even more. Another thing to look out for are her smash attacks. Avoiding that shouldn't be too much of a problem.
When Peach floats she leaves herself wide open. Ness' game is usually up in the air. So if you can get her high enough, you can attempt uair, PKT, PKT2 or whatever you feel is neccessary as that point. If you decide to use PK Flash, don't do it near you or she'll just use Toad and counter it. If you want to share what Peach is doing that gives you most of the problems during the match, just tell me. I hope this helps.