In kpop the word flop is definitely overused and also used from fans to hate on a group and to shade.
Considering a comeback "flop" became something subjective from different points of view.
Excluding nugu groups that are called like that for a reason, just because a song didn't chart on melon top 100 and underperformed compared to other comebacks it doesn't make it automatically a flop when other metrics exist to perform well in a comeback.
So, when do you consider a comeback "flop"?