For being literally just a mixture of dirt and water...

[Your Majesty]

Mud is oddly scarce. I haven't found the Jungle or Desert yet, either. I've explored the whole surface, and they simply aren't there.

I haven't seen everything the game has to offer, yet, so maybe I'll hit the motherlode somewhere underground. Are there any "underground biomes" like in Terraria?

I am a little disappointed that of all the countries I found on the Exploration map, they don't all cover the scarcer resources. Sure, I can get infinite copper, iron, gold and salt, and if I really want a Berry Orchard, I can just import a few in, but... It feels a little bad that I can't get EVERYTHING on a single save. That might just be how the game works, though.

[Mouse Lover]

Yes there are underground biomes.
You can find mud in jungle biomes or places near water.

[Your Majesty]

Alright, good to hear. I haven't mapped out the underground, yet, so I'm hoping some will show up. I've got enough water to keep grain, grass and flower farms going, so now I just want to turn everything topside into nice, orderly stands of trees.

I've found plenty of water, so far; it's all just had sand on the bottom. Here's hoping, anyway.

[FreedomFighter]

Mud is rarer than gold or crystal. Save them for your garden.

[redhongkong]

nvm i found some surface "fish pond" with mud around it.

if u insist on getting mud fast, u can always start a map and ESC-capture mode to look for map with surface fish