Are power chords just for heavy rock and metal songs?

Heavy Metal guitarist rocks

No. Although mainly associated with heavy rock, power chords are used in many other genres too and can also be a good introduction to playing full barre chords.

Listen to old Beatles songs like Helter Skelter and Hey Bulldog for good examples of pop songs that can use power chords.

What is your favorite Power Chord song? . . -:
Power Chord Trainer 3
How to play a power chord of E (E5)Power Chord Trainer - RedPower Chord Trainer with instruction Leaflet