I think she's taken an interest in Carl because he's the one who's most like her. They're both psychopaths. Everybody else, including Frank, has more of conscience than her.
Thank you! I totally agree, it's the only thing that makes sense. She even made that little dig about him not being too bright, which makes him all the more desirous to please her because he wants to show her he's not the idiot she thinks he is. His being held back likely has more to do with his behavioral problems due to his mental and emotional state from his parents rather than any defect in his intelligence quotient, if you could call it a defect.
The fact that he allowed her to call him dumb basically shows just how adrift Carl has become and how much he truly wishes to please an authority figure. I'd like nothing better than for her to just be gone already because Jimmy, dishonest as he has been, actually has a conscience and loves him as much as he loves all the other Gallaghers, and so would make every effort to show Carl he's NOT what Grammy said, in a manner that's not destructive to not only the world, in general, but Carl, himself, which is incredibly important. Fiona's, unfortunately, always too busy and now so is Debbie. Lip is spiraling downward toward his own hellish drain, Ian's finding love with Mickey, Kev and V are involved in missing Ethel and mourning her having run away - the list just goes on and on.
No one has ever had time for him or made true time for him on a constant basis until now and it's the last person he truly needs to bond with or know because she'll be the catalyst that'll send him straight to juvie. Even Debbie's more grounded than she once was, but Carl's still got no one but the worst to cling to, like Lil' Hank and Grammy, who are more mirrors of who he'll be if no one continues to step up and step in for him past talking. He can't do it all himself and someone needs to realize that soon or he'll be permanently lost.