I was thinking she does overact a lot of the time.
So far it's only been in The Sensorites that she seems to be anything more than an Earth child. At first I thought maybe she was adopted.
On a side note, I was looking through a list of episodes and came across The Sontaran Experiment which I recognized as something I'd seen on TV when I must have been five or so. I guess it must have been the first episode of Doctor Who I'd ever seen, and I had no idea what it was. I can't wait to get there in my watchthrough.