Example: Why can you look at Medusa in a mirror and not be turned to stone?
It seems to me that such questions, unless the mechanism is explained in the myth, are purely speculative and not definitively answerable. Are such questions on topic?
I think they should be considered on topic.
The idea that this type of question is bad, and shouldn't be asked, unless there is a good answer described in related material doesn't seem really actionable to me. That implies that an asker must know the answer to the question before they ask.
If you have done the research to reasonably conclude that the phenomenon has no documented explanation, then I would say write an answer saying as much.
Hopefully that fact that the cited question actually did get a reasonable answer proves to be the rule rather than the exception.
I would vote no, because all this type of question can generate is a bunch of opinions. Closing as "primarily opinion based" would be appropriate.
The exception being those cases where it's explained in the myth itself or in a related myth.
One example of a question that would fall under the "close" and do so quickly (which it was) would be How Enkidu was created out of clay and saliva?.
The example of Medusa did, indeed, get a perfectly reasonable answer, and honestly, I think that question was probably OK. It could be argued that the answer didn't really answer the question, it just pushed it back. the answer was "It was looking directly into Medusa's eyes that would turn a mortal to stone, not the whole of her face." The question still remains... Why only directly? Why would a reflection not petrify you?
The answer still falls short of providing a mechanism, which is at the heart of this meta question.
There are perfectly reasonable possible reasons that looking at a reflection would be "safe"... A reflection in a polished shield isn't perfect, and the distortion/imperfections could nullify the effect, etc. But again, that's all just speculation.
We should probably wait and see. On several StackExchange sites, there's not a lot of concern for THE RIGHT ANSWER. Several of them accept plausible options. If that's the case here, I'm OK with that, but if we're going off the StackExchange guideline that real questions have answers, this should probably be considered.
Example: in the Medusa myth, you could ask "Was this explained, and if so, when and how?" That would be enough to salvage it and make it answerable by something other than opinion.