can give some insight being an oli main..
pkfire isn't as good as some make it out to be. if olimar is playhing the mu wrong and throwing out way more pikmin than he should, then it can be effective. but if olimar doesn't side-b as much, pkf goes back to being bad. it's good for landings and edgeguarding tho.
pikmin-lagged attacks are actually something that we look out for and try to generate for easier punishes.
nair is very good in the mu since it gets pikmin off ness's entire body, it beats usmash, and can be effective in edgeguarding.
dair is kinda ****ty for gimping due to whistle but if you get a read on a jump or w/e then it can be cute. ness's other aerials are better.
don't land on top of olimar cuz uair > everything ness has. land away with a fair or a nair.
grab is good. grab a lot. throw him offstage or use it to get him in the air.
blue bthrow isn't as good of a kill move as some may think. purple uthrow is much better. blue bthrow really shouldn't kill until high percents if you know how to DI. the kill strenght for throws is
purple uthrow (kills around 120%) > blue uthrow(kills around 140) > blue bthrow/fthrow(kills around 150, mainly used for damage anyway, so may be stale.) > yellow uthrow (kills around 180%) > red uthrow (won't kill till like 250 lol. )
also when you get olimar in the air focus on getting his landings and forcing him to the edge. some ppl may get super greedy once he's in the air, then overcommit and let him land, turning a good position into a ****ty one.
also keep in mind that olimar has no grab armor so if you're close and you think he'll grab just jab him or ftilt or w/e.
and finally... frigate isn't a bad olimar stage and delfino is one of our best. it's kinda an '08 mentality to think that because theres water, it's a good cp against olimar XD. that stage will give us a full line of purples so fast you won't know what you hit you. (EDIT: That being said, delfino is a solid pick against olimar if you're not too bothered with extra purples. The potential spike opportunities can tip the tides in your favor, but the same can be said for olimar. basically everything he can do on that stage you can do as well.) i'd suggest a small stage that doesn't have many platforms, aka smashville. brinstar, of course is probably the best cp in that mu.
frigate CAN work sometimes, but keep in mind there is a greater incidence of purple and yellow plucks, the first transformation can give you as many problems as it does olimar, and the second phase is godlike for olimar.
PS2 (arguably olimars worst stage), Brinstar/RC, and Smashville would be the best places to go.