Usagi treats her like dirt. Maybe that's why she's a brat? I mean, how would you like to be called a fungus and be yelled at all the time? It's a fact that Usagi would make a poor mom at this point. Maybe she's a better mom in the future when she actually gives birth to Chibi, but not now. I think Makoto would be able to take care of Chibi-Usa better.