A Rocket Loans personal loan does help you build credit, as long as you make the monthly payments on time. If you do make payments on time, Rocket Loans will relay positive information to all three major credit bureaus each month, which will improve your credit score.
You can try the free credit score simulator on WalletHub to get a better idea of how a Rocket Loans loan might impact your credit-building efforts in particular. Just keep in mind that a Rocket Loans loan can also hurt your credit. The company will perform a hard inquiry into your credit history when you apply, which will likely lower your score by about 5 - 10 points. Your score will also drop due to any payments that are 30+ days late, as well as the loan adding to your overall debt load.