Everybody is judging Finn for calling her 'princess'. They don't realise that even in that moment, his final moment he was thinking of her. He called her princess, to make a moment as loaded with emotion as that, somehow easy for her. It was not only to tell her that it's alright but it's also a homage to the past. To them. How he began to know her as princess and how he ended it the same way. 'Thanks princess' is to tell Clarke that he loves her and that she has nothing to regret..