I think you misread the "humorous, but not surprising" remark. It wasn't about credulity in believing in flea circuses, but about the well-known Victorian tendency to apply human emotions to animals. What the author found "humorous, but not surprising" was that this tendency extended to insects. Also, costumes were little bits of tissue paper. I think a lot of the interpretation of what the fleas were doing came down to the showmanship of the guy running the flea circus.