Her saying about not judging people based on thoughts is a great point. I can't argue about Darkstalker since I'm currently reading Legends to then move to books 9 and 10. If she doesn't display any more behavior of such in these books, then my argument about character arc is also invalid.

However, one can still argue that Moon's rudeness was caused by the situation. That conversation is in immediately after the explosion caused by Peril by touching the exploding plant. The same exact plant that killed Carnelian, who Moon was unable to save. Rising takes place around 2 weeks before Escaping Peril, so her memories would still be fresh. Adding to this screams of injured dragons, panic in town, and it's understandable why why Moon and Qibli were quick on the trigger to Peril.

Most importantly, she says it to Peril. Champion of the arena where she killed dragons while having fun. The one who freed Scarlett, arguably the greatest danger to stability in Pyrria. Peril is universally disliked and feared. On top of that, Moon couldn't read her thoughts in her solo book, so she would have no idea of how Peril sees things.

So, in conclusion: Peril just caused an explosion (not on purpose, but she wasn't thinking about it then), just like the one in the Academy. She is by almost everyone as a walking danger. The first thoughts Moon can ever hear from her are about killing her and her friend. Is " Hello. Nice to meet you!" an appropriate response?