There are plenty of anime characters who are widely beloved - but not all of them deserve that adoration. In fact, a significant portion of protagonists are unbelievably overrated. The buck doesn't stop at characters, either; there are plenty of overrated anime programs, as well. Some overvalued heroes fit snuggly into established tropes without adding a new or interesting spin to them. Others are just compromised characters who did horrible things that were ultimately ignored by the fandom.
Overrated doesn't necessarily mean bad. Actually, some of these characters are awesome. Overrated means they get more attention than they deserve, or that their flaws are overlooked. If you see your favorite character on this list, it doesn't mean you have bad taste or that you shouldn't love them - only that there are good reasons others may not feel the same way.
Though few people would count Ash Ketchum among their favorite anime characters, everyone knows his name. He might be the protagonist of a franchise that introduced a whole generation to anime, but he's not all that interesting himself. Ash is so notorious for his lack of character development that he literally doesn't age - but he also barely has a personality, to begin with.
How does one describe Ash? Naive? Cheerful? Those traits have less to do with him as an individual, and rather are pretty general descriptors for someone his age. He's super bland, which is fine if you want to imagine yourself on your own Pokémon journey and forget he exists - but not so great from a character perspective.
Kirito is one of those odd characters who is simultaneously overrated and underrated - he has haters who write him off as completely worthless, as well as devoted fans who think he's flawless. Actually, Kirito is just a regular shonen protagonist, neither especially interesting nor egregiously terrible.
He has moments of interesting development in Sword Art Online II, in which he actually begins to process the trauma of being trapped in an online world and forced to kill people. That said, his personality is inconsistent, and he's overpowered to the point that it feels implausible. Not to mention he's just an unremarkable guy overall.
Goku is the beloved protagonist of an anime with serious nostalgia value, the Dragon Ball series. While Goku isn't a terrible character, he isn't as compelling as some of the other Z-Fighters.
The series puts little effort into his character development; his personality stays constant throughout the entire series. For those interested in his increasingly badass and formidable fighting prowess, this may not be an issue - though they might consider him pointlessly overpowered. Those who want to see him grow emotionally will find themselves disappointed.
We've seen the tormented, revenge-seeking protagonist trope too often in modern anime. While there's nothing inherently wrong with Eren's fiery personality, it's been done to death. He's the star of a show that resonates with a lot of viewers for its inventiveness, which makes the generic nature of his character hard to overlook.
It's fair for him to want revenge, but it's an obvious - and therefore less compelling - storytelling choice.