"What she said." = "That's right" ???
I saw this below.

Girl A : Don't mind us, just get changed.
Girl B : What she said. There's no male locker-room after all.
Boy A: There's no way I could do that!

Question

"What she said." = "That's right" ???
I saw this below.

Girl A : Don't mind us, just get changed.
Girl B : What she said. There's no male locker-room after all.
Boy A: There's no way I could do that!

Answer

"I agree with what she said", or "What she said is true".
So, yes, roughly it is "that's right".

Answer

@Chris Thank you very much!

Answer

Gary's answer is completely correct.  Here, "what she said" means "I agree with her."
However, you should be very careful with this structure.  Often, saying "that's what she said" is used to turn an innocent sentence into a dirty joke.
A: In order to succeed, you need to work long and hard.
B: That's what she said. [because male genitalia are long and hard]