Is it bad to join the editorial board of a journal on Beall’s list?
Joining an editorial board is a form of endorsement as well as a service to the community.  You should never become an editor unless:


You understand how the publisher and journal operate, and you have good reason to believe the operations are competent and professional in every way.
You know the other editors, at least by reputation, and are certain that they are actively engaged in running an academically respectable journal.  In particular, you have talked with other editors about how the journal is doing and what is involved in joining the editorial board.
You honestly believe that publishing papers in this journal is good for the research community as well as the authors (and no well-informed person could describe it as junk, corrupt, predatory, or exploitative).


If you know enough to be sure all three conditions hold, then it doesn't matter what Beall says.  If anyone questions the respectability of the journal, you can convince them they are wrong.  Over time, the reputation should improve.

If you aren't sure, then you need to investigate further.  You have no business joining the editorial board of a journal you aren't prepared to endorse.  (I'd even go so far as to say it's unethical to lend your reputation to a journal that doesn't deserve it.)  If the journal is predatory, then being an editor will look bad.

When I've joined editorial boards for journals I know well as a reader and author, I've still had discussions about expectations for editors and how the journal works from the inside.  Becoming an editor is a substantial decision that should be based on careful consideration.  Nobody will be offended if you have questions or just want to talk.  (At least, if they are offended, then you shouldn't trust them.)

There's also the question of whether becoming an editor of a low-prestige journal looks bad, assuming it's not predatory but just publishes below-average papers.  One key question is whether the journal publishes papers you are interested in, papers you or other people you respect consider worth reading and citing (even if they aren't exciting or important papers).  If so, then being an editor sounds worthwhile.  If not, then what's the point of being an editor?  If the papers aren't worth reading, then listing it on your CV risks making people think "This person either has low standards or is willing to do pointless work just in order to be an editor."  That's not a disaster, but it's not a particularly flattering assessment.