Has john cena lost at WrestleMania?

yes although He has been defeated at WM but he's never been pinned at WM

Won against The Big Show at WM20

Won against JBL at WM21

Won against HHH at WM22

Won against Shawn Michaels at WM23

Lost but not pinned against Orton (winner) and HHH at WM24

Won against Edge and Show at WM25

won at WM26

Lost at WM27 to the miz