You need to be more defensive, most of the time in between Marth's attacks you could be shield grabbing instead of standing still or running into him.
Be careful when edgeguarding too, Olimar isn't good at it but you tend to jump into the fray without hesitation, the last thing you need is to get gimped while trying to gimp someone.
Also, pay more attention to your line because most of the time you just attacked with any pikmin which is risky because of the random results(upsmashing with white -_-).
Like the others said though, you need to use the pikmin throw and the pivot grab more as they are two of the most important tools in Olimar's arsenal.
EDIT: Make use of the whistle armor as well!!!