Do they have kiosks to print boarding passes?

Yes.

Yes, they have kiosks to print boarding passes. The Frontier counter opens two hours before the flight the departs; it's on the right as soon as you go thru the main entrance. Here is the link to the Airport for all other information. http://www.mercercounty.org/departments/transportation-and-infrastructure/trenton-mercer-airport

If you're checking a bag you have to go to the counter anyway. The counter opens an hour before the flight leaves. Smallest airport I've ever been to. They have 2 gates.