I wouldn't know, I haven't ever played a glacier map. Though... getting mud would be nearly impossible anyway.
Mud's not essential. (In fact, can't think of the last time I've
had to muddy ground. 40d-era might have been the last time. Soil will work for crops. Aboveground
and belowground, according to the already mentioned lighting statuses... And I tend to channel into soil for belowground Z-levels but with aboveground light levels, because it also gives a surface devoid of those farm-plot interupting stones and other non-soil squares, giving a predictable area that can
all be devoted to farming.
Anyway, at least part of my glaciers have (under at least one layer of natural ice-block, somewhere on the map) had a soil layer. Either that or I'm digging under a corner of the map that has snow-covered soil (may include trees growing in it), in order to get my soil floors. And if exposed to daylight (by chanelling from above, and even before natural snow-covering started) it went from being able to grow underground plants to not being able to grow anything at all. (Which I take as meaning that it would be aboveground plants, except that no above-ground plants worked in the tundra-like biome. Certainly there were none growing naturally, just the trees. I suppose modding one of the in as being tolerant of arctic-conditions might have proved this theory.)