How do I make a thin layer of lava for mob farming?

In order to create a lava trap that damages monsters but does not destroy items, you want to keep a ratio of 1 bucket of lava to six blocks of trench.

This will allow the lava to burn monsters (and you if you don't make an obsidian skin potion!), but will not be deep enough to destroy the drops -- even Coins are safe.

I didn't have a bucket of lava handy, but this is what your moat should look like - though I had to use water, which is significantly less lethal.

The last thing to note, is that your liquids may get, for lack of a better word, "stuck" on the edge of the trench. In that case, all you need to do is exit / reload the world, which will settle the liquids, and appropriately depth your trench.

A bucket of lava contains one "square unit" of lava - if you pour it into a trench that is longer than 1 block wide, it will spread out.

There's a maximum amount of spaces it will flow over before it just "evaporates," so it might take some experimentation to determine how many buckets of lava you need in order to fill an arbitrary width trench to the depth you desire.

You can test your trench by tossing in something that isn't worth much, say a copper coin or a piece of wood or dirt. If there's lava in the square where it lands, but the item survives, you've hit the right depth.

I have more experience with lava in older versions of Terraria, but in previous versions it was possible to fill a bucket with less than a full unit of lava. You can use this to duplicate lava, for instance. Pour a full bucket out over a 2 or 3 block wide trench, and then before it completely settles, click the bucket on it again. You'll leave a half a block of lava, and gain a full bucket's worth.