I think the term MAMIL was first coined to describe those who got into cycling in middle age as a fashion thing more than anything else, and bought all the racing kit to look the part.
well it was a marketing term originally I think, some market research firm found there was this group of middle age men, in the top social grades, with a decent income, who were spending alot of their spare money on expensive bikes & cycling stuff (as opposed to buying Italian sportscars, motorbikes or golfing) the lycra bit wasnt even really that big a part of it though inevitably they were kitting themselves out in early years Rapha I guess, back to the got a decent income and spending money on expensive things.
but then we had the London Olympics, the boom in cycling, and the tabloids sort of twisted it into this term to what they called "the weekend warriors" to begin with, as the rise of sport cycling (as opposed to purely utility cycling) took off at the weekends especially, and then just anyone on a bike in anything that remotely looked like sporty cycling kit. thesedays it gets used as a perjorative often to body shame people, often just to out group them as different.
really as long as the people riding the bikes are happy wearing whatever they choose, then really it doesnt matter what anyone else thinks of them.