Marth's aerials in neutral are actually really fraudulent. You can just shield then punish because they have no shield stun and it's like -800 or some ****.
If you can read when he's going to jump and fair you can just run under and shield and then grab him.
What makes the matchup terrifying is when marth has space to wd back ->fsmash, dd grab, or dtilt.
Pichu has to be incredibly patient and mostly gets little pokes to gain stage control because marth is a low tier once he's pressed up in a corner.
I want to talk more about some things I've been thinking about with that mu recently.
Personally, I'm a big fan of FoD and Battlefield in the matchup these days.
FoD is a bit scary because of the sides and the low platforms if you get caught on them since it's a free tipper. The ceiling is also a little high for my preference, but I like how limited marth's movement is on the stage, and pichu actually still gets a lot from the low platforms and how low the bottom is, which allows you to go pretty deep with dairs through marth's upB and stuff at lower percents.I feel like it's one of the harder stages to learn against marth, but I like it a lot.
I'm not a fan of dreamland against smart marths because of the amount of space he has, but it also gives you a lot more space if you don't want to get smothered. You live a little longer, and since pichu get's most kills off gimps, it doesn't hurt your kill power too much (although it still does a little), but marth with a lot of ground room is never fun. It's cool that he can't fsmash you for recovering to a platform though.
As any other character, I don't mind yoshi's against marth, and I think it's a super overrated stage against him, but as Pichu it's really scary because Pichu just isn't as good on yoshi's and dying at 20% actually comes into play way more. I like to think my Pichu isn't the worst on yoshi's, but that mu is pretty brutal if you make any mistakes. The bottom is also super high up which can be a problem, but unlike battlefield, you have walls beneath you that marth can tech on to recover. In general, I try to avoid it.
I used to be a big fan of stadium, but I really dislike not having a top platform to retreat to. It's also got a lot of space and close sides which can be a problem. Low ceiling is nice though.
I have some more stuff to say too, not just regarding stages, but that matchup is absolutely garbage and one of the only matchups I really don't enjoy as Pichu.