He shouldn't, it send a bad message out there. What is his job anyways?
Why shouldn't he? And what kind of bad message?
As for his job - he's the President of Poland and making such gestures is one of the things that presidents do.
dared to apologize for Jedwabne in the name of the Polish nation. (...) He could do it only in his own name
Not really, Polish presidents are being voted in into the office in democratic elections and have the right to represent the Polish nation... Just like Yeltsin did when he apologised for the Katyń massacre:
I suspect you don't have a problem with such a gesture in that case?
And, anyway, I don't get it what's your problem - Duda didn't apologise for anything in the name of anyone. He just put a wreath and said a prayer for the victims - it's the most basic, humane gesture...