To put it most simply, an unlikable character can be a good thing, and is definitely encouraged for certain antagonists and even some protagonists, but an uninteresting character is every reader and writer’s nightmare.
There seems to be a terrible problem of people who do character analysis and top 10 worst lists who really don’t understand this. It fill their lists with “worst” characters and they are all villains. Now I’m not saying you can’t put villains on any of these types of lists, but when a handful of them are all the major villains there’s definitely an issue, unless the writer just really sucks at antagonists.
A “good” unlikable character is one you love to hate. For example, Scar from the Lion King. He doesn’t really do much that can be considered morally good in the movie, but he still has personality, layers and a scheme which makes him a character that can be enjoyed.
Opposed to an uninteresting character (mainstream examples are hard) who doesn’t have many interesting traits. Maybe they’re a protagonists, maybe they’re an antagonist, but nothing really stands out about them. Or maybe they just make bad decision for no reason and have no fun dialogue and nothing interesting to contribute to the plot. This would be considered a “bad” character.
It is not necessarily the character who does things you hate, like kill your favorite character and makes a few bad decisions. I find most often it is an “important” character who takes no risks, makes not consequential mistakes and is just basically bland.
So next time you want to do a top 10 worst, or least favorite list, remember to keep the words, “because he’s evil” and “because he’s stupid” out without expanding on what you really mean. Instead of “I hate Ashfur because he was so stupid to attack his leader”, say “I hate Ashfur because he does attacks his leader without any build up or explanation and his character arc was very rushed. He jumped from a normal side character to Judge Frollo in one scene.”