Well, it does capture his personality. But that letter might be a little extreme even for Draco. I always thought he acted a little more high-and-mighty than he actually feels (b/c it is expected of a pureblood to do so).
I think that Draco would realize on some level that if a muggle-born is a genius, then her muggle parents have to be some kind of smart, too. Although he was probably taught by his dad that muggles are idiots...
Author's response
I always viewed Draco Malfoy as the sort of person who has pretty much been raised to think he can do anything he wants and get away with it, because of his family's status. This causes him to pull a few really boneheaded stunts even in canon that he SHOULD have realized there was no way for him to get away with -- dressing up as a Dementor during a Quidditch match, in the hope of getting Harry Potter to faint, and somehow not realizing that it might be a bad idea to do this in front of the entire school?!
Draco isn't stupid by any means, but he does have this tendency to think he's untouchable. This is probably the reason for his breakdown in HBP, when he seriously has to face up to reality and realize that not only is he not untouchable, but he is not the Big Bad Slytherin he's always viewed himself as... and what's more, he's beginning to realize that Voldemort is just viewing him as cannon fodder.
If this story takes place before HBP, then I have no problem in accepting Draco as the sort of person who would write this letter and send it thinking there's no way he'll get in trouble for it. After all, he's just making his opinion of Muggles (and of the assignment) known... and if you're going to insult those worthless Muggles, you might as well go to the top.
I think he should be glad that he didn't send the letter to the Queen. ^_^