Isn't "sour" used for describing "tastes"?
Indeed it is! But it can also be used to describe facial expressions, relationships or even moods. In this context, "sour" means unfriendly or easily annoyed. If someone says that she is in a sour mood, it means that she is crabby or upset. Ex: Arguing over money can turn any relationship sour. Ex: Their relationship turned sour after he lied to her about his past. Ex: He gave me a sour expression when I told him that I didn't like the meal he prepared.