My players never respect the NPCs

This may contain spoilers of Lost Mine of Phandilver

Currently I playing "Lost Mine of Phandilver" and after saving Sildar they keep calling him stupid (out of character), the same goes to Gundern Rockseeker even though they haven't saved him yet. They pulled Sister Garrel out of Phandalin for the quest she gave them and she been called stupid as well.

so far the only ones who they haven't called stupid are the shopkeepers

I don't know how to deal with this, no matter what I do this has to happen, I feel that it is disrespectful to me because i am running these NPCs to them, and I am trying to make them suitable to them as much as possible.

I set a rule for them to use our native language for OOC talks and English for In Character.

-Edit: Since many asked for it I would farther explain how they feel about Sildar and Garale

One of them finds that Sildar is pathetic and useless and he has no reason to stick around while the other view him as a Mercenary still didn't get the third's Opinion.

about Garale, the first doesn't know about her enough to give an opinion, while the other finds her to be a coward.

-TLDR, My players are disrespectful to the NPCs OOC though, how should I deal with that?