I'd argue that asking about misconceptions, whether accidentally or on purpose, is important. That's not what I disagree with here. (I actually think one of the questions tagged with quantum-mythology is pretty decent - the one on the quantum bogo sort - because it asks an interesting question about why the algorithm doesn't work and what we can learn from it from a quantum computing persepective.)
I think it's perfectly on topic to ask why an idea about quantum computing is wrong, if one is clearly asking in good faith. I also think that removing questions explicitly about misconceptions produces an unfortunate juxtaposition - if someone asks a question in which they are clearly thinking incorrectly about the subject, that's a misconception which I think most here would agree should be corrected in an answer to the question (and which I think most here would agree is on topic) - but misconceptions in general are supposed to be off-topic under our rule. So that bans a large set of good questions from the site.
So, yes, these questions should be on-topic.
The real problem here is that quantum-mythology as a tag is honestly kind of terrible. It uses a term that is not academically used (i.e., I can't think of anyone who would think of a misconception in their understanding of the field as a "quantum myth" as that conjures up the image of Zeus in a lab trying to put a quantum computer together, which is...ehm...well, obviously not right). This makes it harder to find the correct tag. Further, the style of the tag would drive off experts. If an expert comes and finds this tag which is not commonly used and easily brings up the wrong idea of what it means, they'll think this is a crackpot site and run away.
Finally, misconceptions is just a simpler, more commonly used term for what we're talking about. So if we were to have a tag for this, it should be misconceptions.
Should we have a tag at all?
I'd argue, though, that we shouldn't have a tag at all, for one simple reason: this tag is a meta-tag. It doesn't convey much useful about the question, and a lot of questions could have it depending on your definition.
The purpose of a tag is to help categorize. If a tag can have a varying amount of questions in it due to the vague definition, then it can't convey much information about the tag, and therefore shouldn't be used. This on top of the fact that the tag would be a meta-tag, which is discouraged to begin with, makes me think that a tag called quantum-mythology OR misconceptions would be a bad idea.
Tl;dr: These types of questions are on-topic but should be tagged normally, without any special tag.