Mocking someone’s accent just tells everyone you speak fewer languages than they do.

Someone speaking your language with an accent is doing something in their second or third language while you do it in your first. Mocking it advertises exactly that gap, and everyone in the room does the arithmetic instantly. It also lands on something people cannot change quickly, which is why it sticks for years.

Do not join in, and if it is easy, say the flat version: that is not funny. Then ask them a question about the thing they were saying before they got interrupted.