Okay, this has been on my mind since I saw this episode, but think about it. What if someone you love came to you one day and told you that they'd made a replica of you, only "cooler" and "more supportive"? Wouldn't that hurt very personally? I think that's why Dipper hated Dippy Fresh so much. It hurt him to see that his twin sister had made a replacement Dipper because real Dipper wasn't cool.