I hope it does poorly. It deserves to.
I don't get this line of thinking.
Let's say it does really poorly, like Green Lantern levels, this would then impact the production of some other justice league movies, and also the superhero movie regular releases we get every year or so. This means less comicbook movies are being made, fewer DC properties. The more of these that are made, the bigger the chance that a few or several of them turn out great and that DC can, as a result, learn as a lesson on how they should approach the different characters, but that's not happening any more if it does not do well.
Now, some of us may not like this movie, but there are others who do. But now we are all getting fewer of these movies as a result.
As someone who didn't like this movie, you can talk about how much you hated it and what you hated about it and all of it is equally as valid as someone's else (who loved the movie) opinion of the movie. The existence of a bad movie adaptation does not in any way impact the source material it was based on. That is still there. Superman/Batman you like and know are still there. Heck they're making a DC animated movie of the killing joke this year, and it is possible because superhero movies are doing so well. As someone who likes comicbook movies, when they are done right in the way you want it, your best outcome in a situation like this is that it does well enough to warrant more DC properties being put in to production. You should be gearing towards: 'I hope it does well, and that they learn from their mistakes, or at least consider some of the complaints some of us have had concerning this movie', which is entirely fair and reasonable and good, because the more people can enjoy a single property, the better it does and the more we all get out of it.