i think the ending is happy ending . look at doona when she open album of picture while 2 of her friends are sleeping n the lyric tell " stay together".

ML and FL are both at great performance.
Idol FL at the start of the drama was at lowest point of hopeless and ML made her spirit life come up again.
At ending, she and her boyfriend together again and kiss.
Her 2 friends also still together and her carrer are good.
so its a trully happy ending...........................