He is a difficult proposition. If he is too attractive, the reader won't understand why the main character ditches him to go out with the true hero. If he is too unattractive, the main character will look like an idiot for staying with him when he's obviously not the one for her.
I read one book where the author had tried too hard to make the Other Man undesirable. He called his girlfriend, the main character, Sweetums. Well, it was obvious she was going to have to ditch him, but the penny didn't drop for her for at least three hundred pages. That meant I lost sympathy for her. Another author tried to subtly make the Other Man unsuitable by having him giving the wrong gift to the main character, but only achieved in making the main character look grasping and ungrateful.
I think the best way is to write an Other Man who is attractive and has no obvious giveaway flaws, but who shows by his actions that he's the wrong man: he lets the main character down in some way, he reveals one of her personal secrets, he tries to take control, he has some questionable views. In other words, there is something that is a deal-breaker about him. If he reveals his flaws through his actions it means he can reveal them quite late on in the story, which is more satisfying for the reader and doesn't make the main character look stupid for thinking that he could be Mr Right.
But it's hard. Everyone has their personal quirks - I know my Other Men tend to be alpha male types, which is often Mr Right in other books. Out there, there may be readers who are baffled at the choices my main characters make or else berating me for making it so obvious who the main character is going to end up with. All I can do is be as aware as I can and make the Other Man as attractive as possible, while not outshining Mr Right-for-her.