It's like 55/45 Olimar overall. Olimar wins through playing offense. Actually camping and playing keepaway in this matchup doesn't work very well, since Mario shrugs off Pikmin easily with D-air and Cape. You're better off rushing down Mario with Smashes and aerials than you are camping in most of this matchup. Once you hit Mario above you, you can juggle him with grab for some nice reward.
You generally don't want to have to shield against Mario, which he will try to force through spacing his fireball approach, since it beats pivot grab. Shielding against Mario is extremely bad for Olimar in this matchup, since he doesn't really have many options to get Mario off once Mario gets in. U-smash oos here and there is alright if his spacing isn't on point.
The other thing you don't want...don't let Mario get a lead in this matchup especially on platformed stages. It's not particularly hard for Mario to win this matchup via timeout if he acquires a reasonable lead on a stage like BF. Just doing basic things like jumping out of shield, B reverse fireballs, Olimar loses a LOT of options if he lets Mario camp on a lead. On the flip side, Olimar always has to make sure he's playing on point, because a mistake anywhere on the screen can result in a comeback if Mario goes for a clutch juggle combo into a gimp that works. Speaking of Mario's juggles, they're obviously really good on Olimar given how floaty he is. Like Luigi, Mario has legit Jab cancel combos on Olimar.
All in all this matchup at its basics will break down into an intense spacing war where Mario and Olimar are competing for the space to create safe approaches that they can get massive reward off of. Very fun matchup for the most part. Liable to take at least 5 minutes if played properly, but a lot happens.
Stages that Mario does better on: PS1, Brinstar, Delfino. All of them have pretty favorable platform layout for the most part for Mario. PS1 has a lot of space, which is something Mario likes in this matchup. Brinstar isn't huge, but Mario has the 2nd best platform control in the game on Brinstar (only bested by Metaknight), and the lava rises are more helpful for Mario than Olimar. Delfino tends to have odd terrain that works better for Mario than Olimar.
I think that covers most of what matters.