I think I've figured out a reason the fans reject Aisha and Nex as a couple. Unlike the other couples, they haven't had many romantic scenes. They've never said, "I love you." They've never kissed. (They were about to on their date, but Squonk interrupted them.) Their romance feels weak.
I blame Nickbow. In seasons 1-4, the couples were more affectionate with each other. We often saw them kissing, cuddling, hugging, etc. It felt more appropriate for their age group.
But from season five onward, the romance has been toned down. There's not much cuddling anymore and almost no kissing. When Aisha and Nex became a couple, the most affection we saw from them was holding hands. That's pretty juvenile.
Also, between Nex and Roy, Roy seemed more romantic. He gave Aisha gifts, he bowed to her, he complimented her, etc. That's the classic idea of the "perfect boyfriend." (Nabu was like that, too.)
Nex doesn't act like that. (He does in the comics and books, but not everyone reads those.) Because of that, the fans think his love for Aisha isn't "real." They think he's just playing around with her, or he just thinks she's pretty.
If Aisha and Nex are a couple in World of Winx, I think Rainbow needs to amp up the romance between them. They need to be as affectionate with each other as the couples were in the early seasons, and Nex needs to act more like that "perfect boyfriend" ideal. Maybe then, the fans will believe their relationship.
(If you're wondering why I didn't post this on my blog, I don't feel like I can without people getting annoyed. I talk about this couple a lot. Maybe I will still post this on my blog eventually.)