The idea that the Beach Boys are "overrated" is subjective. Who is doing the rating and who is doing the reading will make all the difference.
There are personal likes and dislikes that can't be judged or accounted for
completely, even by those who hold them. These basic things strike us in a
certain way, or another.
But practitioners of music, people who work in the industry won't say of the Beach Boys "overrated," for the simple reason that the catalog is so diverse, and some of the stuff is among the most innovative and "significant," to quote
Leonard Bernstein music of its time. Don't Worry Baby, Warmth of the Sun, Pet Sounds and SMiLE, for a few examples. The entire surfing and car group of Top 40 songs too, were unprecedented. To the millions who love any of these
styles, they will always be under-rated, for they have given deep emotional
solace and/or musical challenge.
Composition, arrangement, orchestration, performance and perhaps most of all, recording production boundaries have been pushed aside by Brian and some
of the others, conferring innumerable new artistic and technical satisfactions across the entire popular (and even jazz and classical) business landscapes.
So "overrated?" Not in the impartial musical sense. Only the personal.
Beautiful Dreamer is a wonderful documentary about this phenomenon.