It wasn't as bad as his reputation suggests, but he wasn't a good ruler either. Basically, if you're replacing a leader by force, you have to portray them in a negative light otherwise people will get upset that you've overthrown a good leader.

In the case of Nero, the popular opinion of him is that he fiddled whilst Rome burnt. This isn't true, he did play a role in trying to stop the fire. He had a fondness for theatre, poetry, and music and it was easy for his rivals to say he was too lazy to do anything during a moment of crisis in Rome. The great fire of Rome cleared a large area which is where Nero built his Golden Palace, so he was not only lazy, but he had an agenda in letting Rome burn. We now know this not true, put the myth is still with us.

I'm sure someone can provide a better indepth answer.