Against Mario, one avoid staying offstage, get back up on the stage as soon as possible or else, your going to Cape/FLUUD and there goes a stock. Anyways to be more in depth, most Mario's like using aerial moves to rack up damage, due to his mediocre range on the ground. You can easily Spam Pikmin, but his d-air can get them off in like 1 second after they latch on. He can also Cape, and use them against you, but don't use Pikmin Toss right in front of Mario. Olimar can easily edgeguard Mario with fsmash, d-air, etc. Just again be careful when recovering. One major advantage Olimar has is his grab range. It's amazing, and disjointed, perfect against Mario's that like to be in the air. That'll be your major damage output in this match. Olimar does have a bit better range then Mario so he will use Fireballs/Cape to reverse you around and punish you. But that can help him sometimes. Fireballs can easily be Powershielded/'Swatted' away. Olimar up air has insane priority and will always beat Mario's aerials unless if he spaces them. I think is Up-air, and Down-air can really help against Mario.
Summary: Shield grab, stay off the ledge, use high priority moves, and quick aerials to prevent Mario from juggling you, because once he get's in your range you may be in trouble.