Can I make lemon curd more sour/tart after it's cooked?

Yes, it is possible to make the curd more tart after the fact! I don’t know all the boundaries but this procedure worked for me with the specific recipe in the question:

1. Add 1/2 tsp corn starch to the juice of 2 lemons in a small bowl and mix well.
2. Over a double boiler, bring the finished curd back up to temperature while whisking occasionally.
3. Add the lemon juice/corn starch mixture to the hot curd very slowly, whisking constantly. I added about one teaspoon at a time at 5-10 second intervals. The curd will thin a bit as it takes on more liquid.
4. Continue whisking while holding the curd at 165 deg F until it thickens again. It will not thicken very much or very quickly - this step took me about 10 minutes. This time and temp are not specific targets, just what I measured with a wall clock and Thermapen, for reference.
5. Remove from double boiler and let cool at room temperature. It will continue to thicken as it cools. You may be able to refrigerate it immediately, if your container can take the rapid cooling; I chose to play it safe.

Now, most people will not be working off the same recipe I used. I suspect the fact that my recipe already had two egg yolks contributed to its ability to hold the emulsion during this process. I cannot guarantee that this method will work in all circumstances - but unlike with some sauces, this is provably possible to do with lemon curd! And it now has enough tartness to be worth using.

Here's an image to illustrate the texture of the curd after correcting the tartness and cooling back to room temperature:

It achieved the same thickness/texture as the original.

I have used both citric acid (food grade, sold for canning and jam making, not the descaler) and ascorbic acid (vitamin C) in similar cases when only the acidity of a dish was insufficient. Sometimes lemons are just not sour enough.

The powder comes in very fine texture and can even made finer with a mortar and pestle. A very small amount will go a long way and that’s why it won’t affect the ratios of a dish and a can be stirred into even thick preparations like lemon curd. You can gently warm up your curd so that the butter in it softens, if necessary.