As
@GrimHackers said. You won't find any publicly available addon that automatically awards a trophy. Either you do one of the following:
1. Manually award member of the month every month.
2. Get someone to develop such an addon for you. (definitely a price to pay)
3. Look into the scene of XenForo addon development and create the addons yourself.
This is something I've benefitted from quite a lot.