Captain Olimar has been in the series since Brawl, I've always been on a neutral stance with him. I've neither liked or disliked Olimar. This holds true more of the same for Ultimate. He's from the Pikmin series, as the star of Pikmin 1 and Pikmin 2.
Unlike most characters, his alternate costumes will have a different character with Alph, one of the heroes of Pikmin 3. Makes sense now to me, I was wondering who Alph was and why he was that important to do so.
Both Alph and Olimar play the same. Pretty much all their moves revolve around their Pikmin. You pick them up out of the ground, and mostly throw them at your enemies. I do like that every type of Pikmin causes different kinds of reactions and damage to your foes.
His Final Smash is End of Day, where he gets into his spaceship and crashes into the stage. While it's tough for enemies to dodge, it's not as powerful as you'd think.
When Kirby takes Captain Olimar's power, he gets the power to throw Pikmin. Unlike Olimar and Alph, Kirby is not limited to where his Pikmin have fallen nor does he have to dig any new ones up. If you're fighting Captain Olimar, Kirby is a good way to counter him. He's better than Olimar as Olimar.
Overall I didn't hate Captain Olimar but I didn't love him either. He's a one-trick pony with his Pikmin attack moves. I have seen a good number of players who actually are really good with him, so there's that.