Really. The first book wasn't, IIRC, humorous....so, it gets funny? I wouldn't have thought that would have happened...the first book didn't give me that hint; now you've gotten me more intriqued...
The first book is serious, yes.
Humor is interspersed throughout the rest of the series.
And some stuff is unintentionally funny, like the were-wolf in Prince Caspian at the mound of Aslans Howe.
(I found what he said to be quite funny.. but that's me.)
Reepicheep is a well written character, and Lewsis description of him is good.