I can see what you mean. Wish I wasn't at work right now as I write this so that I could try this out.
What brought this all on, the realization, rather, was the other night over at a friend's house and he was playing guitar on Rocksmith. He knows that I've been playing bass for a few months so he asked if I wanted to play together - him on lead and me on bass. I figured it would be cool so I borrowed his brother's bass and we started in. It literally went something like this:
Played Young Lust - I got destroyed
Player Brick in the Wall (1 - 3) - I got destroyed
Played Pour Some Sugar on Me (arguably the easiest song in the game for bass) - I got destroyed
Talk about walking away in frustration. I was so flustered that after those three songs I just went home.
Were you frustrated because you thought you sounded bad, or because the game told you you were terrible?
Rocksmith is only going to judge you on the timing and pitch of the notes you play, it doesn't care about your fingering or technique. It also doesn't really care if you sounded good or not just that you played what it wanted exactly when it wanted, so I wouldn't take it too seriously if you felt you did ok. Reading the notation will also take practice. If your timing was off on really easy songs you should probably be looking to improve that by learning more about rhythm. Stuff like this: