In our district, I believe a trip can only be excused if it is an "educational" trip. Therefore, a letter such as this one would be perfect and would be required if you have any hopes of getting an excused absence. At WDW, there are specific opportunities for learning, if you choose to accept them. That's the point of the letter, not to hoodwink the teachers. It may be form over substance. It may not. That depends on the parents and the children. Do they take the opportunity to learn something (in a fun way) at World Showcase, AK, etc. or just ride rides all day, everyday?
Personally, I don't see any difference between a trip to WDW and the "educational" field trips taken by our elementary classes. They spend the day at some local museum, animal park, historical site, etc. They make it fun with a small slightly-academic activity but it's mostly just kids having fun out of the classroom. I know because I've been the chaperone on every trip my child has taken. Believe me, it may be "educational" in the larger sense, but not a lot of traditional "learning" is taking place. But I bet every one of those trips is documented on paper as an "educational" trip or there's no way the District would permit it. Who knows, those same teachers who would laugh at your letter may have written the documentation for such a field trip?
To the OP, I don't think your letter was over-the-top. It said what you had to say to get the trip "excused". It didn't go on and on or claim that the only reason for the trip was to educate the child. The teacher had no way of knowing whether you meant what you said or not. She shouldn't have snickered in front of your daughter, if that's what she did. What do these teachers expect? That you would write "We're going to WDW and my child won't be learning a thing while we're gone. By the way, please grant her an excused absence for educational purposes."
Jackie