Daughter jamming on roof

The guitar part is totally what a starter guitarist could pull off and have it work well. all open ringing strings and one note lead bits. She totally played that, although not on the roof top. There were some slide lead bits that sounded accidental but worked that she didn't reproduce on the visuals, I believe the actress played that part though.

My favourite parts of the movie in general are the montage of all the songs they recorded. But definitely the best for me is that scene on the roof, I've watched it on youtube heaps of times.

Why did her mother say she was a terrible guitar player?

The imagining the production of the song was inspired, and hit a real chord so to speak, especially how he acted it, where he wanted to hear certain things and 'worked' it up into what he wanted. Very believable, and illustrates how a producers mind works. I've played half finished tracks to people and all they do is say it sounds rough, but a producer hears the possibility and intention, and ignores the rough.