Depending on your bias, this could either mean that he’s 1) one of the least eloquent and most simplistic speakers in Congress today, or 2) he’s one of the most talented in terms of speaking directly to the American public using clear, easy-to-understand language.
http://www.npr.org/blogs/itsallpolitics/2012/05/21/153024432/sophomoric-members-of-congress-talk-like-10th-graders-analysis-shows (the ranking is at the bottom of the page).
As far as Congress as a whole:
It turns out that the sophistication of congressional speech-making is on the decline, according to the open government group the Sunlight Foundation. Since 2005, the average grade-level at which members of Congress speak has fallen by almost a full grade. … In 2005, Congress spoke at an 11.5 grade level on the Flesch-Kincaid scale. Now, it’s 10.6. In other words, Congress dropped from talking like juniors to talking like sophomores.
The article appropriately points out that this level of discourse is still higher, on average, than the reading level of the average American.