They did in season 2. Jamie was locked up and asked who the greatest swordsman he'd ever seen was and he said Sir Barristan. Something like watching him with a sword was like watching a master painter with his brush.The show has never bothered to convey Barristan's legendary strength and heroism.
In that sense it's a fitting (but depressing and stupid) end for him.
Granted, they never touched on it since. Would have been cook to see that in the preview recap to start the show.