I have now followed some of the research prompted by some answers to my question. Because of what I learned, I'd like to amend my question and here's why:
Clothes are and were expensive so why did so many men have Mohawk costumes available? Specifically Mohawk. People in Boston didn't wear them in their daily lives.
Then I discovered that Adams and Hancock were tea smugglers. Not only that, the British had lowered the price of tea so that it was driving smugglers out of business.
Tea smugglers might have used Mohawk apparel at night in their business of evading British patrol boats, who would be loathe to forcing their way onto an ally's boat, an act tantamount to declaring war.
Are there any references that mention something like this? (I'm not afraid to be shown wrong or ignorant.)
The choice of costume was very intentional, but it was not for anonymity. u/shudderbirds explains it very nicely in this answer.
Not entirely pertinent to the question of disguise, but with regard to the question of smuggling, I wrote this answer.