Kennedy's comment has since earnt her the title "Scotland's most savage nana" among Gallacher's amused friends and wider Facebook community.
The 19-year-old responded to her harsh nana, telling her: "Aw, aye thanks very much nana!
"Dunno whether to laugh or cry right now. Nothing like a wee self confidence boost eh? Cheers doll."
Friends posted reassuring messages, telling the teen she's beautiful just as she is. Her grandmother then agreed with them.
A photo of Kennedy's comments has been retweeted almost 6,000 times, with some Twitter users confessing their grandmothers had made similar comments to them.
Sophie Carr wrote: "Basically what my nanny said to me" while Kate Hayden said: "Literally my nan."
Since the post sparked such a reaction, Kennedy has attempted to explain her comment: "I'm not very good on Facebook, I just go on there to see what's going on," she wrote.
"I probably should have private messaged it because it was just meant to be seen by her but I'm glad she took it well, I was only saying it in a nice way.
"She's beautiful. It wasn't meant as a dig. I was actually quite upset by some of the reaction because she got some digs, or I did.
"I've got ten grandchildren and one great grandchild and I'm very protective of them all. We are all very close so they all know me too well. I'm brutally honest if nothing else."
Gallacher has also responded claiming she "didn't think anything of it" and that her nana "doesn't hod back ... but we wouldn't have her any other way."