First, let me remind everyone that SkyMaxx Pro and Real Weather Connector don't control the visibility and fog effects in X-Plane. All they do is create and place clouds. The issue here from our standpoint is that X-Plane is representing reduced visibility, but we aren't getting any clouds that coincide with that area of reduced visibility. So once you get above the ground fog and look down in this particular case, there are no clouds sitting on the ground to obscure your view of the ground.
The METAR data for UMMG you're finding online doesn't match what X-Plane was receiving at the time. Here's what's in your METAR.rwx:
UMMG 120300Z 00000MPS 0400 R17/0900 FG NSC 14/14 Q1020 NOSIG
What we changed in SMP 3.3.2 was to look for "vertical visibility" information (the VV003 you saw online) and create low clouds when that is seen. That fixes the case you originally sent in. But there is no VV entry, or any cloud coverage information at all in what's published here. I would guess that the information you were looking at was more current, and if you were to fly an hour later the vertical visibility information that was later added to the weather report would have flowed into X-Plane, and your flight would have looked as you expected.
In SMP 3.3.2 you should expect ground fog to work better when VV is indicated in the METAR. This case is different - all we have to go on is the "FG" (fog) entry, but there is no information as to how thick the fog is, so we don't know how to create the clouds that are behind the ground fog.
I suppose we could just guess; it might not match exactly with the thickness of fog that X-Plane represents, but it would probably be better than nothing. I can take a crack at that for the next update.
Bear in mind though this is a different, very specific additional case you've uncovered. Usually FG is accompanied by some sort of cloud cover or vertical visibility information that we can work with.