I'll preface this by clarifying that I'm not a Mario player, but I still think I can point you in the right direction when facing floaties in general.
A lot of times you will not be able to reach floaties before their hitstun ends. However you can typically reach them very shortly after they get out of hitstun. So what are their options if you are near them? They can use a quick aerial if you are in range, they can jump, or they can jump -> air dodge. It's important to note that very few players utilize the last of those options. You can typically cover all of these options but one.
So like when facing Zard for example, you want to directly below him since he can't put a hitbox out below him very quickly. If you are worried about him second jumping away (the most common option chosen by people trying to escape combos), then you have a couple options:
-You can use a low endlag aerial and conserve your second jump so you can follow them when they dodge this (note, this is not effective if they have notably more air mobility than you, however you can sometimes mitigate the air mobility advantage to a degree since you can come in with momentum from your ground speed).
-You can approach with an aerial that will rise past where he currently is so that you cover the possibility of a second jump. It's important to note that if they have a second jump and choose second jump -> air dodge down -> fast fall, a rising aerial could cost you the ability to get down quickly enough to punish the air dodge landing lag. If they actually choose that option sometimes, you're gonna have to mix it up and settle with a 50/50.
To give you another idea of how to deal with their aerials, against Mario I approach him from the front as he exits hitstun, since his fair is slow and his nair has low range (and his fireballs get eaten by aerials).
There are other options as well. For example, both Zard and Marth (and most characters, actually) are pretty unsafe on shield if they land anywhere near you. So if you decide to not go for the juggle like above, or feel you will not have enough frame advantage, then you can instead stay on the ground and try to force them to land uncomfortably close to you. Pull up a shield when they get near and punish OoS. Platforms make this harder, but not impossible. Waveland onto platforms shortly before you need to (if you commit early, they can try to take advantage of your now limited horizontal mobility to land farther away from you... also they can aerial your shield and fall past the platform if they you are close to a platform ledge and they are on the correct side of you (the side nearest the ledge)).
