But might be worth it mainly for the fan-service of having more products and then at the same time getting at least some money without too much work?
My friend made it primarily to let his friends and acquaintances in the developer community support his family a bit (has to support a multi-handicapped daughter) without it being outright donations - we get something for our money and he gets something as well.
Without delving deep into the fine print, Spreadshirt standard cut is 20%. They write that if you do some marketing for them, then you can get up to 60% cut - I haven't seen what the catch is about that deal
But I guess if going that route, then the price could perhaps be set to something like giving Doc a 50% cut - then the prices would be a bit high compared to market price of mugs, but fans might consider it anyway as an alternative "patronage"?
You might be right with the pessimistic view - I might be right with a bit more optimistic view
It'd have to be checked out a bit more in the fine print of Spreadshirt (or similar alternatives.)