I think it's more that those antis Dany want him to be accepted, nonnie. It’s not about logic, and what they’re saying not making any sense to you is a very good sign. Literally, who cares about Young Griff and finds his character amazing? Yet, there are actually people writing posts about how he would be a good King. Let's be honest: they don't like Young Griff, this isn't about him at all. It's about Dany and the fact they hate her. It's about them not wanting her to be Queen, least of all a good Queen. And it's also about misogyny, let's face it.
Those are the same people saying Dany is arrogant (she's not, but Young Griff is) and listing all her flaws when you just have to read both her chapters and his chapters to know who would be a good Queen and who would be a bad King. They also say that he has experience she doesn't since he was trained to be King all his life, which isn't true. He's been raised to be King, he has allies to support his (false) claim, but he never had to rule people. He literally has no idea what being King implies and doesn't understand the people, unlike Dany who wasn't raised to be Queen at all.
It's actually an important message: none of the five key players had been raised to be a leader (far from it), they're outcasts, and they're the ones who have shown the best leadership traits. GRRM isn't without flaws and his books aren't either, but when it comes to this? He knows absolutely what he's doing. The qualities a good ruler should have that are attributed to Young Griff are the ones Dany have been showing through the books.
There’s also another reason why they support Young Griff and want him to be the son of Rhaegar and Elia: their hate towards almost all Targaryens, who are white supremacists (all Dany stans and Targaryens stans are also racists, by the way) because they’re white and blonde and BIPOC characters suffered and died because of them. Elia Martell? Rhaegar’s fault. Quentyn Martell? Dany’s fault. Dorne has suffered so much because of House Targaryen, and Young Griff will show the people of Westeros how kind he is and will honor his mother’s memory, while his supporters will tell everyone the horrors Daenerys commited and how much she doesn’t care about the people. So, when Dany will come and see how the people love him and hate her, she’ll burn KL like in the show because she’s mad and a tyrant.
So again, it’s not about Young Griff. In fact, they don’t care if he’s a Targaryen or if he’s a Blackfyre. It’s about Dany. Her haters have to rewrite canon and go as far as defend slavery to be able to say she’s an awful ruler, a colonizer, a tyrant, a mad woman (emphasis on woman, please)... Her Satanic Majesty, like in the script of the show’s finale, basically. I wasn’t in the ASOIAF fandom before the show aired (I started to read the books after watching the first season, in 2013 I think), but it’s not new. The show did make things worse though: for them, it confirmed their smart theories and how delusional Dany stans are.
Yes, this fandom can be extremely stupid and toxic.