MetaKnight is probably Olimar's worst match-up from what I've seen. Olimar's main weakness is his recovery, and that means MetaKnight can keep him out off the stage and edgehog for an easy win.
Lack of plucking time should not be an issue. You can pluck a full horde of Pikmin in about a second. It's using them that's the problem. He can deflect thrown Pikmin just by spamming A.
So, basically, you have to play it safe against MetaKnight. Don't get off the edge because there's no way he'll let you back on. Stay in the middle and trade hits; Olimar's heavier than he looks, while MetaKnight gets blasted around easily. If you stay in the middle, the best kills are upwards, and Olimar's much better at ceiling KOs than MetaKnight.
Also, stay grounded. MK's air game is lethal (Olimar's is too, of course, but he just can't compare to MetaKnight in that respect). More to the point, on the ground you can get grabs in, and Olimar's uber-long-range grab is one of his best aspects. Back-throws against MK send him flying even at 60%.