It's really hard to guess what kind of person your grandmother was from only one incident.
When someone bad-mouths someone else behind his/her back, I tend to feel that the person who said it has a mean side and I feel I should be wary of that person.
Having said that, you are her grandson and she surely would more openly tell you things that she wouldn't to others, though you were little. Also it's possible that the lodger was indeed an annoying/inconsiderate/horrible kind of person and your grandmother couldn't help talking negatively about her.