If people like her are going to get booed when they leave we can hardly complain about contestants being dull and unopinionated. They'll just want to avoid getting booed.
I suppose that each individual person booing doesn't think they are doing much wrong, but when all the boos are taken together it sounds very unpleasant and embarassing for the show.
I guess that Big Brother fans probably take the whole thing too seriously anyway.
The housemates heard the boos when Lisa left and you can see the reaction to it in this video (taken from the interactive coverage)
http://www.thisisbigbrother.com/gall..._Eviction2.wmv