Practice against players who are better than you an force you to adapt. This is a given for any character but for a character with as unique a play style as Olimar, it's incredibly stupid to underplay the importance of practice. Learn what Pikmin you need for what attacks and how you prefer to play your grab and aerial game, and learn how to micromanage your Pikmin well. Know the standard Pikmin order (red yellow blue white purple) and be able to recite it in your sleep. Play against other characters with projectiles and disjointed hitboxes (mostly swords) and figure out how to play around them and use your range to your advantage. Work on your grab game. Practice recovering on a variety of stages and practice recovering against an edge hogger with your fair or a purple pikmin toss.
tl;dr - Watch the videos posted in the compendium and in Echo's guide, and practice, practice, practice. Olimar is a really unique character and demands a lot of your time. Good luck!