Dipper was in the right. Tandy overreacted at his attempts at a relationship with girls he would otherwise never see again
Dipper wanted to get over Wendy, so he wanted to spread out and talk to other girls his age. He was on a trip, so any girl he may have met would have been out of reach after their initial encounter. He wanted to maximize his chances at a relationship, so he reached out to as many girls as possible. Say he clicked with a girl. Why would any of the other girls react with disgust or annoyance? Dipper is just some guy they met off chance, their feelings wouldn't have been anywhere near hurt. And really, isn't that the risks of meeting people? It's not a guarantee that they will or have to try to meet or talk to you?
And Tandy JUST revealed her feelings for Dipper that episode, and not even to him. She was wrong in reacting the way she did, since that is placing the wrong pressure on Dipper, someone who has never been able to have a relationship with a girl his age like that.