its not so much hes overrated; its just that LOTR is 80 years old. its perfectly understandable if it doesnt age well for some (or indeed, most). his style might be considered overwrought with detail today, but some point between then and now it was considered compellingly descriptive. personally, im the kind of guy who couldnt be concise to save his life, so i can appreciate tolkien, but its very understandable that someone would consider him overrated.
as we know, Tolkien is pretty much the forefather of all fantasy, which means that ALOT of stuff expanding upon his books has been made since then. so, spoiled by more modern fantasy, he just doesnt seem to be nearly as great as he really is. example: General Relativity was a groundbreaking scientific theory when einstein proved it, but its common knowledge today, and is mostly just the groundwork for all the black holes and supersymmetry that occupy modern physics. you have to consider the times to truly appreciate the forefathers; they arent nearly as engaging when compared to decades of evolution in the genre.