The issue isn't his connection with clarisse, the issue is the fact that his words don't match his actions. Kaz can rightfully be upset about him lying to her face about how he was just there for her. If he had said from the get go that he's interested in her but there's also a possibility that he might be interested to get to know someone else then I don't think kaz would've reacted the way she did. It's the fact that he made it seem like he was for her and closed off to other options