fortunately for you, I read this thread on my phone when I'm out of the house, and waiting, and need to burn time.
In today's meta, Olimar revolves around repelling approaches and dealing poison damage while the opponent is distant. Marth is able to approach in such a way that Marth can bridge the distance very easily. Since Olimar's grab is frame 11, Marth can approach falling fair safely. Olimar's shield is subpar, so Dancing Blade is very effective.
Marth's combination of high mobilty on ground, high mobility in the air, and long range makes it so that Marth can consistently break Olimar's zoning, so long as the Marth player does not frequently fall into traps. Additionally, the reward Marth gets for landing a hit is typically massive, because can easily for Olimar to the ledge, where Marth can trap him quite effectively.
However, don't at all count Olimar out in the MU. If the Olimar has superior spacing, the Marth player will find it difficult to parry forward smash, grab, and purple toss. Olimar also has his own options of making reads via dash grab, which is a fantastic bursting move, and approaching Fair, which can hit a jump read very effective. UpB is transcendant, and does not have lag when done near the ground. This causes it to be an excellent way to parry approaches, especially jump-ins. Despite Olimar having a subpar roll, crossing up Dancing Blade is very rewarding. UpSmash kills Marth really early, and between grab and forward smash, Olimar has a powerful ledge trap on Marth as well. Whistle is quite effective on going through Marth's single hit attacks, and letting him get a very rewarding parry, making it very difficult to juggle Olimar. At the highest level of play, the MU is very slight favor Marth, but at high and mid level play, Marth wins solidly. At low level play, Olimar wins out.
Shield breaker is very important in the MU because it will trade with grab, but this will break the gra, since Olimar uses a remote grab via pikmin and never has grab armor. It will also help to break Olimar's shield, which obviously can kill him remarkably early.
It's also important to note that you can shield pikmin toss. Olimars often take advatage of extraneous motions to get rid of pikmin as ways to help make reads. Latched Pikmin can be blocked as well. Unless stuck with a white pikmin, or several pikmin, it's not efficient to hit off every pikmin, as you will get stuck with more in the cooldown of your move, if not punished entirely. Pay attention the the HP of important pikmin, especially purples, which have 15 HP. Obviously, paying attention to the line-up is also important.
For more detailed information, I recommend studying Olimar.