Okay. Been kind of thinking about this on the backburner for the last few hours while doing some house work. Here's how I'd do it:
1. Create a list of monsters, and their relative frequency (Common, Uncommon, etc). Tie those to your various "terrain types" of the underdark. I'd go with something like Aquatic, Dead Caves, Wet Caves, Fungal Forest, Cliff/Canyon. You'll be using these to fill in empty spots in other tables you make. try to make these monsters either unintelligent, or organized in small tribes.
2. When you do tables for a region with intelligent rulers, such as drow, kuo-toa, svirfneblin, duergar, or whatever else, you'll be making tables where about 25% of the table will be centred around these groups directly, and 25% will be centred around the group indirectly. What I mean here is, if you've got 20 entries on your table for drow lands, 5 entries will result in encounters with drow, and 5 entries will be encounters with people that have dealings with drow or otherwise factor into the drow lands (so, driders, escaped slaves, quaggoths, etc).
3. Balance your table how you'd like, in terms of "combat, exploration, social". I'd suggest 50% are predisposed towards combat, and 25% to the other two. But that's just me. If you're doing a bell curve table, make sure that you don't put all the encounters of one type in the same area of the bell curve.
4. Fill in the encounter table with the encounters related to the region you're statting up. At this point, you'll have 50% of the table done. Afterwards, just plug in the monsters you set up in step 1, and you'll quickly have a table put together.
5. Make at least one or two entries something that will trigger a mini adventure. So, myconid colonies, escaped slaves, or a drider NPCs who wants revenge. Stuff like that. Having an entry of "Beholder's Lair" could be fun, for example.
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This is assuming you want a weighted, d8 + d12 table. If you want a d100 table, you can get really fun, and have the tables have subtables like "Ruins", "Fungal Forests", "Nonhuman village", and stuff like that. Go really in depth, and you can run the entire campaign from the tables. If that's the approach you want, check out The West Marches if you haven't already. It has some nice starting points.
