Are you literally seriously asking if jar jar was a fool? I used to believe there were no stupid questions before you asked this.
And, I mean, (almost) everyone else in the Senate agreed to Palpatine's transformation of the Republic into the Empire, so Jar Jar thinking he's right isn't too big of a stretch. And to be completely honest, the way things were at the time, and due to the lack of knowledge about the Emperor's true intentions, the whole Empire thing would have seemed like a great idea, and only those who stick illogically close to democracy or idiots would have thought otherwise.