An Uncle Tron is a stereotypical presentation of a member of a traditionally nerdy or geeky subculture that is meant to both represent or appeal to that subculture. It is more often than not presented in a way that would be outrageously offensive to any other subculture or minority group that society has deemed it politically incorrect to make fun of, e.g., blacks, Jews, or women. Just like other stereotypes, they're presented merely to give the audience someone to laugh at, in this instance, "geeks". This is contrasted with actual nerd-humor from shows such as "Futurama".
The Big Bang Theory is a modern minstrel show featuring Uncle Trons as their main protagonists.
Sheldon: I just but a Nintendo 64 off of ebay last night. Once it gets here I'm going to leave it in the package and argue on the webernets with people about how I'm superior to everyone for owning one.
*Audience dies of laughter, rolling in the aisles, purple in the face*
Sheldon: I can has raeg plz?
*Audience goes into cardiac arrest from laughing so hard*
Sheldon: I've earned two PHDs and have memorized pi to a thousand places but females will forever be outside my grasp.
*Audience slits their wrists and bleeds out in their seats because they've seen the funniest thing ever and now there is no point to living*