User blog comment:Zolnir/RWBY Volume 3: Theories/@comment-25555436-20151016023341/@comment-11188061-20151016024845

Ruby definitely isn't dumb. She just acts cheerful, because if everyone's sad then everyone is definitely sad. So at the very least, she didn't want to appear sad and cause Qrow even more grief (because he made Summer's child unhappy).