I guess his parents really were a cancer to him. Had his mother never shown up, Mochizuki would've never doubted himself and never had a reason to even think about 'what if Kawaru only likes me because I'm a rabbit?' He would've never heard about the bullshit 'dango' thing and tried to turn human. The whole situation is singularly because his "mother" showed up and started placing cancer in his mind. Stupid bitch should've just stayed on the moon and suffocated to death.