(setq my-data '((0 A) (1 B) (2 C)))

(loop for (x y) in my-data
      collect (list y x))
;=> ((A 0) (B 1) (C 2))

(mapcan (lambda (z)
          (destructuring-bind (x y) z
            (list (list y x))))
        my-data)
;=> ((A 0) (B 1) (C 2))

;; (setq my-data '((0 A) (1 B) (2 C)))
(let (res)
  (dolist (z my-data (nreverse res))
    (destructuring-bind (x y) z
      (push (list y x) res))))
;=> ((A 0) (B 1) (C 2))