Truth be told, this isn't a matchup I've seen all too recently, or very often. Plant feels like the easier side to play due to his zoning tools and kit being easier to manage. On the other hand, Olimar with his Pikmin are really difficult to properly handle, but provide a greater reward when mastered. Plant lives longer and has the surprise matchup elements by his side as well.
Olimar, if he throws his Pikmin in front of the Ptooie won't really go anywhere. Also Ollie feels overreliant on his Pikmin for success and his recovery can shift at times depending on how many Pikmin he used.
On the other side of the coin, Olimar can be pretty dangerous if he closes in on Plant and uses his best tools if Plant isn't careful.
The matchup feels pretty even to me, Plant being the easier side to play with while Olimar has the greater reward effects should he play his cards right.
50/50.