I’ve been with my girlfriend for about a year. Normally, she’s quite relaxed, but when she comes to my family’s place, she literally transforms into a different person. She constantly says things like ‘of course, sir’ and ‘whatever you want.’ My parents love it, but I’m starting to feel a bit uncomfortable. Do you think this is excessive or is this normal behavior?
I think it’s quite a normal situation. Acting a bit more formal or polite around families is something everyone does, actually. But if you felt uncomfortable, just tell them directly; they might not realize it. You could say something like, ‘Relax, you get along well with your family anyway.’ 
I think you need to be clear on this matter. Whether you openly like his attitude towards your family or feel uncomfortable, you should speak directly. Instead of saying ‘You’re exaggerating,’ if you say ‘Just act like yourself,’ it would be more constructive 
But wait a minute, why did you get upset? The guy is clearly doing this out of respect for you and your family. I think it’s a bit strange to see this as an excess.
Actually, there may be a need for self-esteem or perhaps approval behind this. In family dynamics, especially in Turkey, the pressure to appear as a prospective son-in-law is very common among men. If it has become uncomfortable, you might consider talking to a professional about it (like a couples therapist).
Let me also add that while being overly polite is not a problem, sometimes it looks so fake that it becomes an issue. My family is very laid-back, and I start to feel like I’m in the ‘ugh, I’m suffocating’ mode, but of course, my family doesn’t notice it 
Well, I think there might be a vibe of “let’s be perfect in front of his family”. Have you ever interacted with his family? It could be a habit or a gap that he inherited from his own family.