Even House of the Dragon's EP is Baffled by Daemon Being Fandom's "Daddy"
Daemon Targaryen (played by Matt Smith) of House of the Dragon objectively is not a good guy, and that's putting it mildly.
At his best, he's solving legitimate problems with liberal applications of violence and brutality, more often he's creating extra problems by needlessly insulting and provoking people, starting with his royal brother. He's not as badass as he thinks, given that his very first major showing at the Episode 1's tourney involved him using dirty tricks and ended with him on his back and at the mercy of his final opponent. And he had murdered at least two people in cold blood, including his first wife, Lady Rhea Royce.
But fans of the series love him. He's definitely one of the most popular characters wherever there is a discussion of House of the Dragon. The series' executive producer Sara Hess even expressed bafflement with Daemon's popularity in the fanbase to THR:
"I'm a little baffled how they're all, 'Oh, daddy!' And I'm just like: 'Really?' How — in what way — was he a good partner, father or brother — to anybody?"
Of course, that sounds a little insincere, given that immediately before Hess admitted to Matt Smith being "incredibly charismatic and wonderful". Since when an actor's charisma was insufficient to make a character popular, despite whatever evil deeds he might commit, particularly if most evil deeds are inflicted upon people who deserved them? (A woman keeping him trapped in a loveless political marriage can count among the "deserving", if you ask certain fangirls!)
Besides, Daemon is an outrageous man in a way that's actually appealing to the audience, flaunting social norms that the viewers are likely to see as oppressive and seeking things that the viewers are likely to see as admirable, like companionship of a woman he loves. And importantly, despite his evil acts, he remains a complex character, whom we're invited to see as a man trapped by society in a bad situation, rather than a man who creates and aggravates a bad situation for his own benefits.
Despite feeling snubbed by his older brother, King Viserys, and clearly desirous of the throne, Daemon never actually plots to take it, and backs off whenever trouble he brewed threatens to go too far, starting with his attention-mongering dragonegg theft. At one moment he even helped ailing Viserys, when the latter stumbled on his way to the Iron Throne.
So we have a bad boy character, who is excellently played by a handsome, charismatic actor, and who is shown to have a good deal of emotional depth to him. Love of the fans is to be expected, if only its extent may be in any way surprising.