Not too bad, actually. The Kirby boards have it listed as 60-40 Olimar's advantage, I personally believe it's a little closer.
First of all, Kirby OWNS Olimar offstage. Dair, Bair, hammer, whatever, he's going to die if you aim properly (superarmor can give you trouble though)
Onstage, Kirby has just as much trouble with Olimar's lame pivot grabs as anybody else.
Final Cutter goes through Pikmin like a knife through butter. It's pretty good.
Inhale him and Starshot offstage for a gimp, or Copy his power and spam the crap out of him. Sure, he can jump and outrange you with his Side B that way, but Kirby has freaking infinite Pikmin.
In the air, Kirby generally wins.
I played a good Olimar on AiB ladder recently (a guy I met in the tourney 2 weeks ago) and I managed to beat him. I played friendlies with him in the tourney, and I think I lost, but I generally don't try hard in friendlies before a tournament >_>
Kirby's neutral A jab is good in this matchup, and Fair is good for hitting Pikmin off...
If you have a good Kirby and know how to fight Olimar, I guess you can win. Just make sure you ban Corneria -_-
http://www.youtube.com/watch?v=9tMYpzdiU6s
http://www.youtube.com/watch?v=Hy4E9aYkZ9M
old videos, back when I had no idea wtf to do against Olimar
All that said, I still despise fighting Olimar -_- he's just so lame...