How are they terrible musicians? They played their specific type of music extremely well. Like, technically better than most people will ever be at music. People loved seeing them play. I still enjoy their records. So, what is terrible?
"technically better than most people"
I guess with them touring and playing basically non-stop a certain kind of that is inevitable.
Their concert frequency was on par with the otherwise known as most prolific band ever, The Beach Boys. It's just that The Ramones' members all died around third of the way (~20 years of touring vs 60).
Maybe it was part of his schtick but johnny often bragged about not practicing. He wanted the concerts to be raw. The mistakes were part of the art.
I played guitar for a while and didn't have much trouble playing them and I was barely adequate at best.
The hardest song for me was California Sun. I just couldn't figure it out until I realized it had 4 chords instead of three. I learned later that it was a cover.