The definition of flop is to fail completely so it is accurate to call BvS a flop, the majority of critics disliked it and so did the majority of the audiences (and Jeremy Irons), it did not make a profit at the box office, it will when they add merchandise and home video but a movie like this shouldn't crawl to making a profit with the help of a bunch of BDs.I don't think many people seriously considered it a flop--it would have had to do much less business for that to have been the case--but it does appear to be a disappointment, yes.
Script -> fail
Direction -> fail
Editing -> fail
Box Office -> fail
Reception -> fail
That it could have been worse doesn't mean it didn't flop. WB wouldn't restructure, switch executives and put Snyder on a leash if it was just financially disappointing. The movie was a disaster considering it was supposed to set the tone for the entire DCEU, it wasn't just a one time thing that can be written of, this was the start of what was supposed to be their biggest franchise for the next few years.
If the Fantastic Beasts movie is a success I expect them to throw money at Joanne Rowling to expand the Potter Universe while a few planned DC movies will be quietly put to rest (who asked for Aquaman and Cyborg for example? And they have a better Flash on tv than Ezra Miller can ever hope to be, so what's the point?)