They both deserve a whack on the head with a rolled up newspaper.... Like on one hand, he botched the proposal. On the other hand, she is really going to extremes to test his love for her. The past 10ish chapters showed she was insecure about their relationship, and had a bit of a flight reflex when she thought their relationship wasn't fully "adult". Tatsu didn't really address this in an clear manner until he was forced to in the last few chapters.
What Tatsu failed to take seriously, the proposal, just put stress on those same fault lines that were present from before. At this point she's going to ask that question that is damnable important in any serious relationship: "What do you like about me."
Of course I still see the cutting of the hair to be a drastic method that probably wasn't called for, but that is just window dressing on the actual conversation they are now going to have. Tatsu can't bullshit or lie here, because this is now a proof of concept. Can Tatsu love her without her hair?