Fight the space man with space Bairs
Use platforms while juggling to avoid Whistle > Nair stuff
Use Pikmin to refresh moves or use GR > Jab String
Kill option from Grab in from GR > Uair starting at 125 if fresh
Try not to fall behind since you'll have to make a read to approach
Bair to Jab mixups > Olimar shield
However, this must be spaced or it doesn't end well. And given that it's dependent on spacing against a tiny target and then being covered by jab mixups, you could see how it could be mind numbing if the Olimar is disciplined with his shield so we'll see this as "low/mid risk to low mid reward" option or the wall strategy. Alternatively, you could also flip jump away after Bair, but I always feel vulnerable after that. However doing so would condition him to chase which opens up the jab game again from shield dropping.
The RPS is roughly like this and this is where you see we lose from the ground:
Olimars Shield to Usmash > Dash Attack
Our Dash Attack > His pivot grab (if spaced correctly since it tends to go over grabs or abuse no grab armor)
Olimar pivot Grab > Shield Approach (or Froll to buffer jab)
Our Shield Approach (or Froll to buffer jab) > Olimar Fsmash (unless he stutter step charges Fsmash)
Olimar Fsmash > Dash Grab
Dash Grab > Shield to Usmash
As you can see, his options are "hit the F out of her", and our options are "poke him". If you're going to win, you have to maximize your punishments off of the pokes without getting too greedy and falling into the getting the F hit out of her punishes on approach.
Olimar's Fair plays in there as well, with something like it loses to our shield but beats dash grab, so most Olimar's I see play the MU don't work too much off of Fair against us as I think their goal is to stay on the ground.
Unfortunately the only times we should be getting Dsmashes in this MU is if we can use our mobility to bait him (which typically requires him overcommitting due to being behind) or a hard read once we get a poke in.
You can use Plasma Whip to lag on pikmin returning to him if you think he's going to try to punish it by dash grabbing it.
Probably a -1 imo
I'd personally go ICs instead, but if I didn't have ICs/Falco to choose from ZSS would be next in line for me to try if I were getting pooped on for some reason