Peach is very, very bad in lag. She might look good at first, but she's really bad.
A Peach player should react well at what the opponent does and punish well, which is why Peach is rarely able to go aggressive all the time. Wi-Fi ruins this. I know this sounds strange, but I can't say it in any other way.
Peach's spacing is VERY important. Ruined. Peach has a good pressure game. Ruined. Not to mention some follow-ups are harder. She is very vulnerable to anti-aerial attacks. Her ground game sucks. Float canceling is very hard to do well (though this is not a huge problem for most but I do use it quite often myself, so for me, this is pretty bad). Her ledge options get worse. This all result into much, much worse match-ups.
Dair really isn't that unpunishable. However, that's not the most important thing, the point is that it's just hard to pressure with Dair. So even if you connect it isn't as rewarding at all, not to mention she reacts worse to DI, something she can usually follow and punish. Her lagless aerials is her only good thing, but that's not that good.
I know many characters get nerfed on Wi-fi, but I think she just gets a much worse treatment. At least, this is how I feel.