No, I think the voice just gradually developed a higher pitch after a while. There's lots of video footage of the Family Guy actors recording, and she just sounds normal.
I think I read on his blog how in the original series, he pitched it up using analog equipment, and was happy with how it sounded, but on the new series he had to do it digitally, and wasn't happy with how it sounded, so they just left it unpitched. I can't imagine there being that much of a difference, but I guess there was enough for him to decide not to use it.