Noticed that this is your first post! Welcome to the boards. Anyways, to answer your question, spacing, baiting, and reading are going to be the tools you use against any problem that you come across in Smash Bros, regardless of the version or title that you're playing. I used to have a lot of trouble against Marth, Shulk, and Ike players spamming the counter function, but now I find it amusing how simple it is to outsmart those kind of players. Doctor Mario is very similar in this sense.
So of course, the first thing you're going to want to do is keep your spacing. You probably already do this as a Marth main, but it's important nonetheless. If you manage to be spaced far enough away that your tipper would hit, chances are his cape isn't going to be able to reach you. When you're spaced like that, you will be able to punish him for abusing that almost instantly. Also focus on when he's using it: is it everytime you get close? does he use it more when he's off-stage? Actively figure out when he's using the cape, and make a mental note of it. Finally, learn how to bait the attack. Generally players who abuse the same functions are stuck in the habit, and will abuse that function even when they know they shouldn't. Buffer some attacks, wait for his reaction, and then punish. It's not easy at first, but if you actively space, bait, and read his play-style, you'll find a way around it. Best of luck!
