It could even be sugar-coated with nice words if necessary.
Migrating users don't receive a lot of guidance on how to manage migrations unless they look for it, so we should be providing that sort of information to them when they perform a migration.
I'm basically stealing ChrisF's commented suggestion from one of Jeff's answers. It's something I'd quite like to see implemented, and I thought to bring it to the wider stage. First, here's the comment:
Can we get "Never migrate crap" (or similar) added to the migrate dialog? I know most people won't read it, but at least if it's there we can point to it as visible documentation of this rule. And if it stops a few people posting that first migrate vote then it will be worth it.
Now, ChrisF obviously makes a very good point that many people won't read it, but at least then we can point them at the UI and say "look, you're not paying attention, please don't do this again," and hopefully at least then stop some repeat offenders.
Low quality and zero-effort questions that get migrated seem to be (based purely on observation, and with no real evidence) much more likely to just sit on their target site abandoned (that is, with no owning user account)—even if they are strictly on-topic on the target site—because often the person who has posted it really doesn't care all that much.
Migrating users don't receive a lot of guidance on how, what or where they should migrate, unless they go looking for it on [MSO] or the privilege pages, which we can only expect a small proportion to do. As such the only real opportunity to educate these users about the migration system is when they are performing a migration, it's the only time they'll be paying attention and be interested in migration, so a few guidelines or rules (but not so many as to put people off) subtly mentioned during the migration process could help guide these people and improve the quality of migrations.