It's not quite the right word because she genuinely, plausibly believed I (the property owner) would consent to having my lawn mowed. So if you want to split hairs, we might say that vandalism requires disregard for the consent of the property owner. Note that we can't reasonably assume that someone who spraypaints my garage door or windows or etc is similarly acting in good faith because it's commonly understood that spray paint is difficult to remove and that it's not widely appreciated (I hope we can agree that 'street art aficionados' are fewer and farther between than people who want their lawns mowed--if we can't then I don't see much point in continuing the thread).