Maybe it was effective to some degree for their people since they were probably trying to brainwash them already with other ways. I do not know for sure but would not say that it had no effect at all whatsoever, because it might have had an effect of some sort on their population.
I believe that the Bromberg Bloody Sunday was a staged affair whose purpose was to portray the Polish people as brute killers without a conscience. Supposedly the Nazi Ministry of Propaganda staged the killing of German soldiers to look as though the Polish men in the small country town murdered them. I believe this event served to deepen the fear everyone felt for the Nazis.
Given that the Germans used Bloody Sunday as an excuse to kill a large swath of the Polish population I assume you could say it was useful propaganda for them in justifying their pursuits. When considering this event it is important to remember that scholars are unsure of all the details involved with this incident.
Yes, I think that Bromberg Bloody Sunday was effective propaganda, because it was part of the sales pitch to the Germans for the war. Germany also played up the situation, by exaggerating the devastation, the number of casualties, and even who was responsible. Germany was successful, because their people were convinced, and it brought the nation one step closer to the Holocaust.
No, I do not think that this event was propaganda at all, and was never tried to be put off as that. This was a real event, where a lot of people were killed. Anything that was shown about this event was true and accurate, and was meant to show what had happened.