That vision I imagine pertains to one's eye sight. And likely to one's artistic abilities. Would we nonetheless deem it a miracle if say the quality were a work of that of Rembrandt (he didn't have the encounter as Saint Faustina had)? Or does it not deem venerable elevation because it lacks the quality as that of Rembrandt? Oddly, Rembrandt painted illustrations/portraits from the "poorly" (this is to emphasize poor in lack of every artistic detail) of the Gospel (say that of the Prodigal Son)? If Rembrandt who had the gift/quality/talent to paint remarkably well beyond Saint Faustina's talent (not making a snide remark at all about her), but an honest comparison, if he could paint better without the Divine Instruction to do so, why would anyone argue the veneration of Saint Faustina's portrait? It doesn't take a miracle to make a masterpiece.