What is with this counter attack vs Olimar discussion?
You'd be much better off using Nair.
Considering that you can counter a PIKMIN, but OLIMAR will still be able to move, shield, dodge, GRAB, etc... the hitlag during counter initiation takes a lot longer than Nair. Nair also leaves you much less open for ending lag. You can also follow up your Nair with Utilt, Bair, etc... you can't follow up a counter. Nair will also get the Pikmin off of you.
Only time I counter Pikmin is when Oli is far away from me and it's a type of Pikmin I want DEAD, not just off of me. Or when I want to look cool. But don't try to look cool when you're up in Oli's face. Looking cool doesn't always win matches.
BTW, don't just panic and try to kill the little leeches as soon as they latch on to you. You're Ike, and you can take more damage than Olimar can. If it's going to mess up an approach, don't get rid of the Pikmin yet. Lots of approaches will get Pikmin off of you anyway. It's best to eat a little damage and possibly get Olimar in an unfavorable position.
(Also, if it helps, I think this match-up is actually one where it's viable to use dash attack as an approach. Make him start throwing stuff at you and dash attack in unless he throws a purple.)