I understand the need (or desire) to inspect boarding cards upon entering the aircraft, and have no problem whatsoever with showing mine when requested.
However, while I would emphasise that there is no excuse for rudeness, cabin crew should remember that by the time that we, the passengers, have reached the aircraft we have been herded like sheep, made to wait in countless, endless lines, patronised and - above all - have probably already shown that bit of cardboard to half a dozen other people on the long journey from departure drop-off to aircraft. Sometimes the insanity of it all just gets us down, and it is perhaps not surprising that our apparent lack of enthusiasm at being asked to show that same bit of paper yet again may be mistaken for grumpiness.
Airports are miserable places, and flying is a miserable experience. Sometimes we can't help but show our feelings, no matter how hard we try not to.