I've also heard the phrase "frowned face" before. Then does the word "frown" imply a negative nuance?
Yes, the word "frown" does have a negative meaning associated with it. When someone frowns, it usually means he is upset, sad, or angry about something. Ex: His smile immediately turned into a frown when he heard the bad news. Ex: Let's turn that frown upside down! (This is an expression that means to try and make someone smile)