If the door was manufactured and not fabricated on site, the likelihood that it is not square is small, especially one with a glass insert. Take a string and run it diagonally from corner, diagonally, to corner. The measurement will confirm if true or out of square. (opposite diagonals of a rectangle will have exact lengths.)
Typically what happens when a door is not square is it is not been mounted properly in the frame and not the door itself 'out'. I learned a trick from a home inspector a long time ago that stuck with me. If you have ever seen a 'wonderbar', it is a framer's crowbar. Take the end of the wonderbar and insert it under the frame on the side of the door that is 'low'. step on the bar until the frame raises to 'level' with a spirit level. It does not have to be perfect, just within your tolerance of sqaure.
With a tiny house the larger issue might be the foundation is not level causing the house frame to flex. If you find other doors that are not square of windows that are 'sticky', you may need to level/shim the foundation.