This suggestion is intended to make small scale farming and "touchups" more practical compared to mass irrigation by flooding. I'm sure almost everyone here has had to create mud at one point or another. Underground this is required to grow crops on stone floors and on the surface there are scattered tiles of stone that restrict how big you can make a surface farm plot. Since most interactions with terrain tiles such as digging, channeling, engraving etc are designations you can use to "paint" the job command onto the intended tiles. I think it would be useful to have a command to add mud to specific tiles. This would be much easier than the game's current solution for mudding single tiles which is to dig a hole in the ceiling above it, or building a wall and a ramp so that you can create a pond zone and get dwarves to drop water from above down onto the floor below.
How this works is pretty simple, you just tag the tiles you want to add mud to as if smoothing or engraving the floors. This creates a hauling job where dwarves will first find a bucket, fill the bucket at a designated source of water and then carry the bucket to the tile where they dump the water onto the floor. This creates a 1x1 tile of mud after the single layer of water evapourates. You can designate as many or as little tiles as you want, however the limited throughput of buckets means that you would be better served using pumps and other methods to irrigate large areas. This job shares the same type as filling pods or pits, so if that task is disabled dwarves will not muddy tiles.
At the same time as the above command you could add a second one for cleaning up mud on stone tiles (which the current cleaning task does not handle). For instance if some water gets dropped halfway up your nice stone staircase that tile will be muddied and begin to grow cave moss. Cleaning a tile works the same way as mudding the tile, a dwarf carries a bucket of water to the intended tile and after some "smoothing" the tile will be clean and free of mud and other growths.
In both cases when multiple tiles are to be muddied or cleaned dwarves who take up this task will "reserve" more than one tile and immediately refill their bucket to work on the next tile after the first tile is done. This prevents dwarves from wasting time repeatedly bouncing buckets in and out of stockpiles if you wish to work on a large area.