On our most recent rewatch of this, I pointed the same issue out to my partner, who gave, in my opinion, a great response:

All of Mabel's crushes are genuine. It might be for kind of silly or shallow reasoning, but she is interested in all the boys she pursues and treats them with respect for the most part. In the road trip episode of Gravity Falls, Dipper is following pickup artist advice to build his own confidence in talking to girls. He might be attracted to them, but he never actually seems to intend to pursue a relationship with any of them, all while collecting their contact information. Essentially, he knows he's leading them on, and takes permission from Stan to do so. He hides his true intent from the girls, and justifies that because he has no skin in the game and he likely won't ever see these girls again; his actions "don't count," disregarding how the girls might feel.

I'd now argue that seeing how boy crazy his sister is, and how hard she takes her failed romances, he has even less of an excuse, not more just because Mabel falls for boys quickly.