DLC includes different cars, and decorative options. With regards to the cars, there are several different hitboxes that cars can utilize. Here's a spreadsheet that shows the differences. Some cars are flatter than others, some are taller, and some are a little bigger. It's not an outright advantage, and that's why you haven't seen a lot of people treating it as pay-to-win or anything. But a tip I can give you as a Rocket League veteran is that you should try out the different cars and see what works best for your playstyle.
That being said, once you start getting better at the game, you will start to notice a bigger difference in playstyle between each type of hitbox and you'll then be able to reevaluate what car you want to use. Most pros use Octane. It's a great all-arounder. It's what I use. Dominus, which comes in the Supersonic Fury DLC pack, is probably the second most-used. Then there are a few pros who like the Batmobile. But yeah for now, don't worry about what car you use. It will come in time.
