> Isn't a band's purpose to produce good music and aren't people supposed to like musicians because they produce good music?

There are two definitions of "good" here, that are different.

1. "good musician" means a musician who is skilled or adept in their instrument. I'm separating this from if a musician is good at writing music.

2. "good music" means music that is entertaining or enjoyable.

The Ramones were not incredibly skilled in their instruments. They wrote music that many people found enjoyable. They were not good musicians, but they created good music.