俺も初心者ですが、こんなので合ってますか。

(defun show-list (lst)
  (cond
    ((atom lst)
     (format t "~A" lst))
    (t
      (format t "[")
      (show-list (car lst))
      (show-rest (cdr lst)))))

(defun show-rest (lst)
  (cond
    ((null lst)
     (format t "]"))
    ((atom lst)
     (format t " . ~A]" lst))
    (t
      (format t " ")
      (show-list (car lst))
      (show-rest (cdr lst)))))