Touko got too impatient and screwed up badly with this conversation, Mayumi's too worried about Rin to really notice the "Who cares."
Seems like we're rapidly approaching the situation where Touko's going to have to deliberately burn Mayumi's country life down to get her away from the Seagull Villa, and what's currently stopping Touko from doing that is that a failure will kill even the slightest hope of any continued connection, not that there was much of one to begin with.