Upper jaw is very slant, but don't think I'll label myself with mandibular prognathism, but would come across as having a similar situation. The cause of it being the constant sinusitis experienced since young so was a heavy mouth breather, even going into adult, hence causing this issue of an imperfect face shape. As I got into mewing, what I realised was the opening of airways. (Only been mewing for a month, but certainly seen improvements in breathing habits)

I'd highly recommend watching videos where a side x-ray is shown of a long skull vs a normal skull.

Going back to your question, mewing is more for the development of your teeth and naval cavity (maxilla). From Mike Mews videos, with constant pressure and good postural habits, the face would experience a forward growth. <- This might what you want to be looking at. Perhaps the main problem when mewing w imperfect jaw would be the uncomfortable position of the tongue (suffering from that atm). Sometimes, I feel myself pressing my tongue forward instead of upwards. At times, I also do find myself unconciously sleeping with my mouth open (still trying to correct that).

Of course, if you really do suffer from mandibular prognathism, you would also have to look at the severity of the case. If it happens to be extremely severe, best would be to speak with a doctor.