You're right, Olimar's air game isn't the best. There are some ways to get back to the ground. If you have a purple Pikmin in your line, you'll be fine. Grabbing the ledge and then throwing a purple one to make the part around the ledge safer is definitely a thing. even if you're somewhere in the air right above the stage, a purple pikmin thrown right can help you out. (if you're enemy tries to get you with a fair or bair for example, you have a situation to throw a purple pikmin at him)
You should also be aware that you have some mix-ups for landing without throwing out any move. A well timed airdodge can get you pass the opponent. Falling more "zick-zack" like may also make it more difficult to predict your landing spot. And saving your second jump to avoid the opponent is also always good. These are at least the options I consider when I want to get back on stage if I'm somewhere above the stage and want to get back to the ground.
If you want to get back on stage when being somewhere offstage near the blastzone, you also have good tools as an Olimar player. Your recovery is great, because you can control it that well. The less pikmin you have, the faster you can fly around and the lower you can go before recovering. You should mix up with your ways to recover. Sometimes recover high to surprise your opponent. Sometimes go for the ledge. Other times you may want to go underneath the stage to the opposite ledge. Or you want to recover and land on a plattform onstage. Also keep in mind you can use ONE aerial while you use your recovery. This makes it less risky to land directly on stage. Still, sometimes the ledge is the better option. You just have to watch how your opponent reacts. Will he try to gimp you? Maybe even get a spike on you? Or will he just try to not allow you to grab the ledge? Will he go for ledge trumps?
Even if you want to go for the most obvious ledge, you still can be less predictable by sometimes flying straight to the ledge and other times fly along the wall to the ledge. I've written more than perhaps needed, but this should show you how many options you have to recover and get back on the stage. Keep up mixing these things up to be almost unpredictable. Even if some people think it's easy to gimp Olimar, I feel like this is wrong. He has such a great recovery and he can even protect himself by throwing out an aerial during the recovery, so it's not like a "punish-me-please" recovery.