I think it's disrespectful on both parts, tbh.
The mother who knew that Avril - whom was entering a drug store - clearly wasn't in the mood to be bombarded with fans and she was clearly going about her business and the mother disregarded that by approaching her in a drug store.
Yet Avril's reaction is also disrespectful, she could've respectfully said she wasn't in the mood or right state of mind to deal with fans right now, but could've thanked them for supporting her or something.