In what way was Brexit a stupid thing? It was an extemely important decision, directly affecting everybody's life. If asking people about anything then isn't that what it should be? You might not like the outcome (I don't) but I consider the question of Brexit important.

Analogy:

Cancer surgery is an extremely important decision, directly affecting many people's lives.

What happened with Brexit was a analogous a bunch of salesmen on TV saying "that mysterious ache you have, don't listen to doctors who say it's fine, call our surgical team today! It's cancer! We can fix this quickly and you'll be back to your old self within a week!" for two decades, then the country agreeing, going to surgery, and waking up to find they'd had half their liver removed, the post-surgical biopsy results said it was fine and not cancerous at all, it took 6 months to recover and they could never drink alcohol again. And the ache was still there.

If it had been an honest "we know it will cost X, we are willing to spend this because otherwise what is the point of money", that would have been totally fair.

Instead, problems that weren't caused by the EU were blamed on it for decades, while the benefits of membership were treated as the natural state of the world to the extent that talk of losing them was equated with "being punished".

Should we also have a referendum on reducing the tax rate to zero? It would also be an extremely important decision, directly affecting everybody's life. If asking people about anything, then isn't that what it should be?

Not all referenda that might win a "yes" vote are sensible to propose.