I'm not sure what motive he has in doing an Eminem diss unless his record company pressured him into it to create controversy to help push the album. I agree though, he's never pretended to be something he's not. I doubt when he said he wants to show his harder side, he meant his "gangsta" side. I think he meant it more as 'I'm a grown man in real life, not the goofy ass Fresh Prince of Bel-Air'.
Like dude said, when you buy a Will Smith album, you know what to expect.