A lot of Top 40 stuff these days is just a melody sung over a simple drum and bass track. I can hum along to a Ke$ha song in my head (although I'd rather not), but the music backing it is totally forgetable.
An example: Royals by Lorde. This is, IMO, a Top 40 song that doesn't suck, because her vocals are great and the melody is catchy. But it sounds like there is no backing band, just a drum machine.
No reason you can't make that into a rock song, and one of my friends did exactly that with his band. His drummer, who also happens to have a great voice, totally crushed it while the band played a rythm guitar riff over it. It went over very well.
Katy Perry and Ke$ha? Well, those songs might not have melodies as catchy as Royals, but you can do this with almost any of these dance-beat junk songs.
I think it was harder with old 80s pop like Michael Jackson, where the music itself made a huge impression on the listener. The bass line from Billy Jean, for example, or Eddie Van Halen's guitar solo in Beat It. The synthesizer riff in Take On Me. Covering those songs actually requires some instrumental know-how.
Now, the problem for rock musicians is that most of us didn't spend years learning to play an electric guitar or bass to play Ke$ha covers; we want to play music we actually like. The gulf between what is popular to the public and what is popular amongst musicians seems to grow wider with time. Remember, in the early 70s Pink Floyd and Led Zeppelin were considered "pop". Horns of a dilemma.