I did find a plain green sweater for $4 at Salvation Army, which I used as the backdrop for my own nerdy/ugly sweater. Behold!
Everything except the lights around the neck and Darth Vader's face was made from felt. It's a great material because there is no fraying, so it can be attached with hot glue or sewn on without having to hem it.
I cut the stars, snowmen, and gingerbread men by tracing around cookie cutters.
The Santa hat was cut freehand, and Darth Vader's face was cut from an old T-shirt I bought to turn into a pillowcase. I left about an inch around the face to prevent fraying.
I added a few more nerdy touches: the gingerbread men are wearing Starfleet shirts (in red, of course). I got the idea from a Foxtrot strip. The snowmen are dressed as the 4th and 11th Doctors. Here's an in-progress shot from before I attached all of the pieces.
I embroidered the snowman faces, but all of the other details are done with Sharpie markers. Both look great on the felt. All of the pieces are glued on except for Darth Vader's face, which I sewed on using blanket stitch.
The finishing touch was my only purchase besides the sweater: a set of Christmas light beads that I found from Oriental Trading Company. They cost $15 including shipping, which is pretty steep, but I think it's worth it.
And one final shot of me in my sweater. The neck is a lot wider than I usually wear, which adds to the "ugly" effect and also shows off the Christmas lights rather nicely.
Happy Christmas, live long and prosper, and may the force be with you.