Typically, that's true. They will portray a character similar to them.Her character, Penny, seems like an ESFP to me. I have no idea what she is in real life. Most actors that I have seen though seem to not have a very large range, as far as types they can portray - usually throughout their roles, they seem to play the same or similar MBTI type characters.
And speaking of Big Bang Theory, I love Bernadette's outfits and would very much want to steal her wardrobe if I were not a head taller than she is. ^_~