All right, I'll try to mount a defence.

I would be modestly surprised if any member of Congress has an IQ below 100. You just need to have a bit of smarts to get elected. Even if the seat you want is safe, i.e. repeatedly won by the same party, you likely have to win a competitive primary. To win elections you need to make speeches, answer questions, participate in debates and so on. It's hard. And you'll have opponents that are ready to pounce on every mistake you make and try make a big deal out of it. Even smart people make lots of mistakes and say stupid things when put on the spot. I doubt a person of below average intelligence even has a chance.

Even George W. Bush, who's said and done a lot of stupid things and is often considered dim for a politician, likely has an IQ above 120.

As for decency and honesty, a useful rule of thumb is that most people are good. Crooked people are certainly a significant minority but most of them don't hide their crookedness very well. And you can't be visibly crooked and still win elections. Your opponents are motivated to dig up the dirt on you.

As for honestly trying to serve their country I admit that this is a bit tricky. Congresspeople certainly have a structural incentive to put the interests of their district above that of their country. But they are not completely short-sighted and neither are their constitutents. Conditions in congressional district X are very dependent on conditions in the US as a whole. So I do think congresspeople try to honestly serve both their district and their country.

Non-corruption is again a bit tricky but here I side with Matt Yglesias and Paul Waldman:

The truth, however, is that Congress is probably less corrupt than at any point in our history. Real old-fashioned corruption, of the briefcase-full-of-cash kind, is extremely rare (though it still happens, as with William Jefferson, he of the $90,000 stuffed in the freezer).

Real old-school corruption like you have in third world countries and like you used to have more of in Congress is now very rare. There's still a real debate to be had about the role of lobbyists, campaign finance law, structural incentives and so on but that's not what I'm talking about here.

Are there still some bad apples? Definitely. But I stand by my view that the vast majority are not.