I was born in 1956 in Brittany and never heard about "Kevin" as a first name before the 1980'. My opinion is this is just because of the explosions of medias that occurred at that time (end of ORTF, massive invasion of US TV shows and series)
Don't you think in the 90s, Celtic first names in general came back? I knew a lot of 'guillian', 'kevin' (and alternative graphie like 'kevan'), and even Bleuen, Marwen ? I felt like my parents generation celtics name were Yann and Patrick, and in mine it was much more diverse.
I never heard of the expression 'kevin' to talk about annoying or oblivious kids until I left Brittany (then internet arrived and everything changed).
It might be because of the invasion of us tv shows, but don't you think there was regionalism also in the area?