Problem with this question is the same as it always is:
-How can you measure the 'ratedness' of the thing in question? (band in this case)
-How can you measure the 'true quality' of the thing in question?
-How can you balance these two factors off against each other?
I mean, for someone like The Beatles, it is pretty much inevitable that they will be considered overrated by a sizeable number of people. Not because they weren't a good band, but simply because they are SO highly rated by millions that simply being a 'good' band may not be good enough; essentially anything short of 'completely amazing' makes them overrated.
My gut feeling is that most supergroups are probably 'overrated', including the Beatles, but I still think they are a great band.
As for the comparisons with Oasis lyrics I'm not convinced. You cite Champagne Supernova but is that really lyrically superior to the Beatles? Bear in mind I'm a big Oasis fan, they were my favourite band for over 10 years, I've seen them live more than any other band and I've probably listened to their songs more than any other artist. Fantastic melodies but lyrically inconsistent in my book.
That's a very good point. Very highly rated things are very likely to be labelled as over-rated, especially by those who aren't fans of what ever it is. I find the Beatles' music to be boring and bland, and I can't understand the fuss people make over them so they appear "over-rated" to me because I see/hear people rating them far far higher than what I'd rate them.