Sure, but years in the war will make a jokester into a hard-ass. Like Ax mentioned, Andalites aren't known for having a sense of humor, it doesn't mean there aren't andalites with a sense of humor, it just means that the population as a whole is mostly serious.
I'm positive that on the home world, there are kids having fun, and families laughing their tails off, but in the military, you're not going to get many funny guys.