[Unfortunately for Mabel, there really isn't a nice way to say that without offending Wirt.]

Well, neither is Dipper! [He stops though, realizing how it sounds about three seconds after it comes out of his mouth.] ...W-Wait, that's not-- that came out wrong. I didn't mean it like that, okay? I just...I didn't think he cared about that kind of thing. That's all.

[Saying Dipper isn't cool isn't so much a knock against him as it is an admission that if they went to the same school in the same grade they would probably sit at the same lunch table and hang out in similar social circles.]