Ness doesn't have the advantage in the mathup. It's in Olimar's favor but nothing drastic, imo. Ness has problems approaching and sticking to Olimar, which is the main thing and the reason a lot of characters have seemingly good matchups against him. Is there anything in particular you think Ness has that puts the matchup in his favor?
As for our point of view, fsmash is an obvious tool, as it is against most characters. Yellow fsmash will pass through your fair, cutting off that approach. From what I understand, most of Ness' approaches are short hopped aerials ending in landing next to the opponent. In these situations, Olimar can do what he does best, punish run away and focus on punishing landings. Fsmash and Grab are great for this. I forget what fram your grab comes out on, but I remember it being on the slow side, when it comes to close range grabs. This is a huge problem for you against us because one of the main tools characters need to learn how to use against Olimar up close is grab when they have him under shield pressure. If this important tool is already sub-par, well, you know.
But yeah, anything you'd like to add from your point of view? I'll stop rambling for now.