Check AMS Mathematics Subject Classification, you will find the 2 digit classification "97" for mathematics education. Thus it seems that It is allowed to ask mathematics education research questions on MO. But, such questions often find the following problems on MO:

They could hardly remain open since basically they haven't got one correct answer.

Since they haven't got one correct answer people often Judges them as CW or "soft". Of course, this is not a problem if the question remains open.

Asking them in way that is suitable for MO is too difficult. Of course, this is a nice challenge for the OP if he or she has a slim belief that the question has a chance to remain open.

Let me give an example. Here is my last question on MO. For me, it is indeed a "hard"
**serious research problem**. Respecting MO rules, I couldn't ask for peoples' opinion about the possible effects of teaching wordy heuristic strategies on different thinking styles. I couldn't ask for references, since I am a mathematics educator and it is my job to know and to find them. And I couldn't ask such and such questions for different reasons. Thus, I did try to set my question in a way that the answers remain based on facts rather than opinions. And I hoped to shed some light on my research problem. But the question is now [on hold]. And I am holding this question:

Is mathematics education research question allowed on MO?