I messed around a bit with Kirby's copy ability and Olimar's Tackle Pikmin Throw in Training and took down a few notes.
- Pikmin thrown by Kirby can get hit by a nonfatal attack, like a white fair or sometimes Olimar's own Tackle Pikmin Throw. In the case where Kirby's Pikmin gets hit, it will get back up, return to the spot where he threw it, and die there.
- Two reds or yellows will just pass through each other. In every other case where two Pikmin collide, it's a little difficult to tell. Most of the time, Kirby's Pikmin will beat out Olimar's Tackle Pikmin, but there were some cases when they just both hit each other and fall to the ground simultaneously.
- Kirby is limited to only 3 Pikmin on the field, like Olimar. This isn't really relevant in the matchup a lot unless Kirby's Pikmin gets hit or if Kirby is throwing them offstage. In the case where there are 3 Pikmin out, the animation still occurs but with no Pikmin projectile.
- For some reason, Kirby's Pikmin do more damage than Olimar's own Tackle Pikmin Throw. (7/6/6/5/9, in order of R/Y/B/W/P, compared to 5/5/5/3/8)
- Kirby's 3rd neutral B, Jumping Inhale, gets a copy of Sticky Pikmin Throw instead, intentionally or not. It does the exact same damage as Olimar's and is easily stopped by Olimar's Tackle Pikmin Throw.
- EDIT: Plucking in the air does nothing, but Kirby's animation doesn't cancel out when landing unlike Olimar's.
In any case, Olimar should be able to keep Kirby out, given that Kirby's most effective only closeup, but I wanted to find a few facts about worst case scenario since I see people talk about how getting the Copy Ability makes the matchup lopsided.