I can see why people hate him, but personally I never minded him, I had a stronger disliking for Jake Lloyd than for Jar Jar lol.
But anyway, sure he is a clumsy person who doesn't seem fully aware of what's going on all the time. But he did manage to unite the Naboo and the Gungans, although by accident, for a clumsy fellow he has alot of luck.
And about the downfall of the republic thanks to Jar-Jar, like others said, he just mentioned the idea, it was up to the other senators to agree or disagree, but they practically unanymously agreed.
For the same matter it might have been Padme who was asked/forced to suggest it if she didn't run away. Let's see the Portman fans bash her if that were the case XD
And as a final point in defense of all the Gungans, and as a pointer to those who hate comic relief characters, not all Gungans are like Jar-Jar, if we look at Captain Tarpals he seemed like a cunning warrior.
But ofcourse most of us define an entire Star Wars species by the examples we get, for example we saw Chewie as the first Wookiee, and from there on we thought that all Wookiees were tough and cool like he was. But for the same matter there might be a pacifist Wookiee somewhere on Kashyyyk hehe.