Yahz is correct, Tacitus is quite firm on the subject that Nero was no where near Rome when the fire started.
It should be noted that fires in Rome were not unusual, it was that this one was much more widespread than most affecting thousands, and that Nero was appalled, he promptly looked around for a scapegoat and landed on the Christians. It would not have been in the interest of the Christians to have started any fires, apart from which it would have been out of character with both St. Peter and St. Paul to do so, and both are thought to have been put to death in the ensuing revenge attacks organised by Nero.
In the aftermath of the great fire of London, many of the Londoners blamed a Popish plot, however as the King would have none of it and was popular and the source of the fire was known, few French and Catholics died.
Nero was not popular when the fire broke out so his enemies were ready to fuel any rumours that he may have been responsible, also his eagerness to build and extend on a truly massive scale his Palace gave the rumour legs.
Tacitus is fairly reliable as he had access to Palace records and spoke to people alive at the time, also he expresses the opinion that Nero was a disgrace to the Purple why?
Was it the Murders Nero commited, the wedding on stage where he played the bride and was Bugg3red in front of an audience, the reign of terror where men of wealth were forced to take their own lives and give their money to him?
No what really upset Tacitus was that Nero became an Actor, it was a different World with different standards, but if Nero had been guilty Tacitus would have been only too ready to point the finger.