I recall seeing someone suggest that we dress up in costumes on Purim merely in order to increase the happiness on this day.
Has anyone seen this?
No.
The custom of Purim costumes can be traced back only to medieval Europe, and is not mentioned before that time. Some have speculated it reminds us that Mordechai was dressed in regal clothing by the king, or that Esther hid her Jewish identity, or that there are costume parties in the story. Others say it started as an imitation of Christian carnivals that took place at that time of year.
At any rate, there is no tie to the Mishna's "Mishenichnas Adar marbim be-simcha -- When Adar comes, we increase our joy".
The Rema even allowed sha’atnez and cross-dressing in the costume on the grounds that there is no direct benefit from it:
As to the custom of wearing 'faces' on Purim, and men who wear women's dresses and women wearing men's attire - this is not forbidden, for they have no intention other than pure pleasure. So too the practice of wearing kilayim, rabbinically forbidden mixtures of clothes. [Orach Hayyim 696:8].