I wouldn't worry too much about it, underage drinking tickets are very common. It's a non-criminal offense so you won't have to report that "you've committed a crime."
"Non-mandatory" just means that you can pay the ticket and not show up in court. Some tickets require you to show up in court regardless of whether you intend to pay/plea guilty.
The only benefit to going to the court date is that you might be able to get the city attorney reduce the fine and/or amend the ticket to something that sounds better than "underage drinking." You don't need a lawyer to do this. As a bargaining chip you could offer to take that Alcohol Awareness course.
Since it's municipal court your ticket will not be on CCAP, which is where most people look. It will likely show up on your Wisconsin Department of Justice CIB record which people can order for a $18 fee.