I learned the word ‘nim-garm’ in Urdu to mean ‘lukewarm’. Is it used in Persian?

Yes: nim-garm نیم گرم ‘lukewarm’. A synonym is velarm ولرم. Nim/nimeنیم/نیمه means ‘half’ and can be used as a prefix. Other examples: nim-faasele نیم فاصله ‘half space’ (in typing); nime-kaare نیمه کاره ‘half-done, incomplete’. As a noun: nime-ye avval نیمه‌ی اول ‘first half’ (e.g., in football).