It's all in the execution and how fully they commit to it. He can still look goofy but be totally sinister. One's perception changes of a thing once it emotionally affects you. Something can look initially silly but when it has actually scared you, you don't think of it as silly anymore. After all, many people are scared of clowns. They're often just people in a gaudy circus costume with white make-up, a red nose and a wig. They could look ridiculous and goofy, but why do people find them so creepy?
And the 70s TV Incredible Hulk could seem goofy and rather B-movie-ish if you simply looked at pictures, but when you watch the actual series, it is really quite tense and like a psychological horror. That version is actually far scarier than any of the more monstrous versions from the films.
So Mysterious can easily have a fishbowl and theatrical costume. On the surface he could look silly, but once he's scared the **** out of Spider-Man (and possibly the audience) that is not going to look silly anymore.