I'm inclined to think that the difference is cultural, rather than biological. Andalite society may simply place less emphasis on humor, and more emphasis on other outlets for clever thinking such as science. Besides that, they don't understand
human humor because things like sarcasm might be culture-specific.
Just look at Arbron from TAC. He may not have been any Marco, but he certainly had his own style of humor, despite being an Andalite. Not to mention that most of the Animorphs thought that Ax eventually developed a very dry sense of humor from being exposed to humans for so long.
there are plenty of humans who dont understand sarcasm either, might not be an andalite thing, just an ax and elfangor thing. aldrea had no issues understanding sarcasm in andalite chroncles, and many of the other andalites in that book did things like calling the hork bajar genius.
I think the 'genius' comment could be called a KASU, since it's not like Ax couldn't hear sarcasm, it was more like he didn't even understand the concept, which implies that sarcasm is unheard-of among Andalites. Actually, now that I'm thinking about it . . . shouldn't sarcasm be impossible in thought-speak anyway, since thought-speak works by images rather than words?