That's misleading. Noone says that we shouldn't have had a referendum because the issue was important. The reason why many people (including me) thought the referendum was a bad idea in the first place is because as a matter of principle, we have a representative democracy - in which we (supposedly) pay MPs to devote themselves full time to jobs that include studying and coming to an understanding of the complex issues involved in Governing the country - and having referenda undermines this representative democracy. In general, making decisions by referenda often tends to lead to bad decisions because most people simply don't have the time to study the implications of different possible decisions - for the very obvious reason that most of us are busy getting on with our lives, most of us don't have degrees in economics, and most of us are working in careers that have nothing to do with Government. So it's nothing to do with the importance or otherwise of the decision, it's about respecting how our representative democracy works.
Sure, in principle, I would never normally support referenda. But we're now we're in a situation where it's becoming increasingly apparent that implementing the vote from the previous referendum is becoming untenable because there's no sensible way to implement it without either doing very serious harm to our economy (no-deal Brexit) or Brexiting with a deal (the May deal) which it's apparent that almost noone supports. In an ideal world, the decision of what to do to resolve this impasse would be made - in accordance with our representative democracy - by the Government and the MPs. But, since the impasse was created in the first place by a referendum, trying to modify the decision without a referendum would look pretty odd, and possibly destroy many people's faith in our democracy. Because of that, exceptionally, holding another referendum looks like it might be the least bad option we have.
