I'd say "probably," but it mostly depends on the scenario. The way to think about it, rather than "on the cover," is that you can't trade on someone else's reputation. So, if one of your selling points is a character someone else is known for using, it'll probably get you into trouble.
It's possible to interpret that as "just the cover," but I'm not so sure that you'd win in court if DC sued with such a narrow reading. The primary concern is that nobody can confuse your product as coming from a more established company, so the "rules" probably depend on the customers.
As an example, Apple is trying to convince courts that (among other things) cellphone owners are naive enough to not be able to tell the difference between products if they're all black rectangles with rounded corners. It wouldn't be hard to argue the opposite, that comic readers or movie-goers, because of reviews and a shared culture, are savvy enough to make decisions based on what characters appear inside.
Mind you, that's not advice for anybody, just pointing out that it could get much more complicated, depending on the product, audience, and likely plaintiff. And in a Disney-like case, you'd probably be forced out of business even if you're in the right, because it's economical for them to drag the case out until you go bankrupt paying legal fees or settle. Remember "Uncensored Mouse," in fact, Eclipse's reprinting of Mickey Mouse comic strips (which had long fallen into the public domain) under a black cover, wrapped in plastic. They probably wouldn't have won in court, but Eclipse couldn't have survived to the end.