I've found a 45 degree angle is a decent spot for approach if he's camping. If you can time it just right you'll land right in his face with an air attack just as his smash pikmin are finishing their animation... or with minimal pikmin stuck to you that can be knocked off with an appropriate quick attack (which is hopefully a follow up to Olimar's face).
Characters with good rushing attacks or big hit box attacks have been easier to use against Olimar (for me) then characters like Fox and Sheik. I've actually done better with Sonic and Yoshi do to their spin/egg attacks then a space animal because they seem to get in his face easier.
Though an Olimar with good counter timing would be really hard to cope with... as in pikmin aren't flying your way until it's going to ruin your entire attack.
Most projectiles are worthless or not worth the effort against him, the ones that stop once they strike a hard object at least. You pretty much just have to knock him around. But if you can pop him in the air, he's a little easier to deal with because he feels sort of like a laggy toon link.
Oli is cake if you can kill his pikmin and pop him in the air before he plucks more. He'll pretty much have nothing to defend himself with, but if he's playing well that probably won't happen.
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)