It's probably beneficial to sit in on the course if you can. It does give a decent high level overview of trauma care, and as with all the merit badge courses its main value is in providing a structured approach to assessment and resuscitation.
You cannot be certified unless you are a physician or licensed midlevel provider. It also doesn't matter if you are certified or not, because the card itself is meaningless.
Finally, whenever you take ATLS, please keep in mind that it is the least evidence-based of all the merit badge courses. In many aspects it is years behind the times, and it generally has a very dogmatic approach. Take any specific recommendations with a large grain of salt.