I don't think it works as a full name: perhaps because it's an adjective and virtue names tend to be nouns:
Joy rather than Joyous,
Hope rather than Hopeful. (OK, my daughter
Beatrice bucks the trend, but in English anyway, there isn't a noun or an adjective in use that people could associate with
Beatrice!) Or perhaps because I'm familiar with it in real life as a nn for
Meredith (f) and in LOTR as a nn for Meriadoc (m).
Are you thinking of
Merry as male or female?