It's impossible to tell from the word itself, as Sue points out; the context is
everything. On its face, the word has pejorative connotations: the prototype "brat" is a spoiled child who whines and complains and pouts when s/he doesn't get things just his/her way. So, byi nature, it isn't a compliment.
BUT, I suspect your bf is trying to tease you in an affectionate and even amorous way though

.
Him: Let's go to that new Brad Pitt movie, sweetie.
Her: (pretending to pout.... girls like to do that, you know) No, it's too cold out and my feet get chilly, and I don't feel like putting on makeup.
Him: (Because she's acting wimpy and girly, and kind of cute in doing so): Why, you little brat face! I'm going to tickle you good for that!
.... you can use your imagination for the balance....who needs Brad Pitt anyway?
