Yeah, that AiB discussion about Marth and Peach was pretty nuts. I'm going to say it's 50-50 now. I have a lot of reasons to this. Marth takes a certain playstyle for Peach. If you don't use this playstyle you are going to get "stuck" inside of Marth and his aerials. Here's how I treat the matchup:
-If he's not close, I always pull out a turnip
-If I notice that he likes to rush me when I crouch to pull out a turnip, I use this to mess with his head. I'll crouch to pretend to use a turnip, then forward smash him instead. It works and is a great mind game. Try it.
-I like to see what was stated earlier : jump, float in the air, float slightly backwards to evade his swipes, then punish. Yeah this quote:
I like to fool Marths and make them counter and then punish. I float to them as if I were gonna Fair them but instead of landing the hit I float backwards (while Fairing), they counter, then I punish. It only works a few times though, so don't overuse it.
-Love to back air against Marth. Short hopping them. If he's not used to the spacing it will mess up his usual spacing habits.
-If you want to avoid the UP B, after you down air, if he shields it, float forward slightly. You will evade his UP b and get a free hit.
-Also, when you land after he shields one of your aerials, roll immediately. this will also get you to evade his up b, and give you a free hit. Then he will grow much more weary of using it and you can instead follow up your aerials with a bit of pressure.
-Do not use airdodge against marth very much. He can punish it real easily, even if he didn't predict your airdodge, because his swipes come out fast, he can bust out another one pretty fast after your airdodge.
-Pupnish forward smash if you shield it by floating out of shield to forward air. If you master the timing, you are, I'm pretty sure, gauranteed to punish any shielded forward smashes.
-Bait marth into forward smashing by appearing to be in tipper range but being ready to evade it or shield it.
Absolutely critical technique to making this matchup one you beat:
-Edgeguard his recovery by floating next to the ledge. When you hear the sound of his UP b come out, let go of your float, you're invincibility frames you gain from grabbing the ledge will protect you from the UP B and he will be edgehogged
Those are some of the strategies I use. I just don't fear this matchup at all, perhaps its my playstyle, but I love to fight Marth.