Just because you don't like it doesn't mean it's over-rated – you just don't get it…maybe…. I suppose those bands/artists considered to be in the top few of all time must be considered over-rated by many as not everyone can get them.
I saw RHCP before they were big here ('89) so I liked that they later had a few hits but now they're mainstream I suppose which is a shame.
I suppose some can't get past Dylan's voice but for those with less simple tastes there is a lot to admire – obviously really because it's all about his music with him.
As for anyone thinking Zappa is boring, well I assume you haven't listened to much of his stuff – the variety is wider than anyone elses by far so boring doesn't make sense; in terms of musical skill pretty much at the top and comparable to classical composers as well as rock artists.