On Wikipedia it says that Jazz noticed the character change and in one episode said ""you know Mrs. Banks there's something different about you" and in another when Nicky is all grown up he says ""So who's playing the mother this year?"
In light of Joaquin Phoenix's (dubious) retirement from acting, MSN Movies has an article listing 10 other actors it thinks should also throw in the towel when it comes to acting. Will Smith is #3 on that list. From the article:
Well at first when I read the title of the topic, I was excited. BUt after hearing what other ppl have to say, i'm not so sure anymore. So now I'm on the fence.
Although this does not justify it, I heard from an interview that he has childhood memories of his father beating his mother, and that he's sometimes still traumatized by this.