Is my dataset unbalanced?
To answer your question no.

The term "imbalance" usually refers to classification problems. For your case, i.e. a regression problem you can only look at the distribution of your target variable. 

If by "balance" you mean them having a uniform distribution, you could argue that they are, if fact imbalanced. However, I'd argue that this is not the problem here. When steering you rarely need an extreme angle, I think this dataset represents this well.

Your goal is also a bit unclear. Are you trying to "predict the steering angle" or  "follow the lane"? If it is the first, I'd first suggest changing your metric. Use one that tells you actually how well you are doing, e.g. MSE, MAE.

A lot of things could be to blame for a poor performance. I don't thing the distribution of the angles is one of them.