overhated: Wesley Crusher (Star Trek: The Next Generation). I think some fans disliked the "child prodigy with special mind powers" arc, and it is easy to make fun of, but it never really bothered me.

underhated: I remember really disliking Mayuri and Luka when I watched Steins;Gate (the anime, unsure if the VN is better). Mayuri was more of a passive object in the story than an active player, with her most significant role being dying repeatedly in a time loop, and Luka felt like a transphobic stereotype in many ways, with the story constantly reinforcing that "she's a dude" despite her actively seeking a female role in society and requesting that the main character change history so she was born a woman.

To be clear, I am focusing on how well-written the characters are here, not on how much I would hate them if they were real. If that were the question, I'd still say Wesley Crusher was overhated (I think we'd be buddies if he were real), but the underhated category would go to someone who I think is bad within the fiction in a way most people don't recognize. Which could cover any protagonist of a police drama.