There is a long discussion (covering two years) as to the pros and cons of including uBlock Origin as a pre-bundled part of the TBB (https://trac.torproject.org/projects/tor/ticket/17569), but no conclusion was reached. In my experience with normal Firefox, adding uBlock Origin gives a huge improvement in browser performance when going to heavy websites (think CNN.com for example). This is more critical with TB due to the slowness of the Tor network. So I would like to add uBlock Origin to my installation, but of course I am concerned that there will be privacy implications.
Yes - adding uBlock Origin would definitely make you identifiable and would make you stand out from every other Tor user. The way the Tor browser is made is so that websites can see if you are using the Tor browser, but cannot distinguish you from any other Tor user. uBlock Origin blocks many trackers which is a great thing, but you can easily detect it by testing if a tracker script normally allowed by Tor and NoScript, but blocked by uBlock Origin and seeing whether it executes or not. Very few Tor users actually use an ad/tracker blocker and so you could easily be detected to be using Tor and uBlock Origin, and could be fingerprinted. Using any Firefox Addon on the Tor browser is already discouraged by the Tor project since it can definitely make you stand out and uBlock Origin isn't an exception. The Tor browser bundle already comes with NoScript and Https Everywhere and also delete all cookies when the user closes the browser. You don't need uBlock Origin on the Tor browser.