>>712590143The closer you get to Mount Doom, the more corrupting the influence of The Ring. It is a form of spiritual pressure. The Ring is sentient to a degree and will corrupt your soul which is how it turns the bearers into Nazguls if they hold it for too long.
For that reason, I believe Olimar could actually do it so long as he was explicitly told to go toss The Ring into Mount Doom. He'd take a lot longer to get there than Frodo, that's for certain, but he'd be incredibly hard to see, and since his Pikmin are taking the brunt of The Ring's pressure he could just replace them when they got too corrupted.