The definition of ubad bro comes from the online gaming world of having either a bad teammate or enemy and then calling them your bro afterwards. It has become really popular since ubad and bro are both often used when talking online trash.
In the automotive industry, upbadging is when you are driving a lower-trim or entry level trim version of a specific vehicle and you decide to add custom badges, decals, and/or logos from a higher trim in order to make your vehicle look faster/more powerful/more expensive than it actually is.
For example: adding Hellcat badges to an SXT Dodge Challenger is indeed upbadging, and definitely the most godawful of them all. This is because a Hellcat V8 can put out 707hp while a V6 SXT only has 305hp. You drive an upbadged SXT and try to race an actual Hellcat, you're gonna look stupid and you're gonna get called out. Don't do it. Either be proud of your ride or save up for the real deal.