How does a towed banner maintain a steady tilt?
Banners have a lead pole to keep the leading edge straight. This pole is weighted at the bottom, which ensures that the top and bottom of the banner remain in the correct place.
By itself, this would ensure the banner remain perfectly vertical. To achieve the proper angle for viewing on the ground, a tab is attached to the top of the lead pole, adjusted to the required angle to push the top of the pole in the desired direction to achieve the right viewing angle.
Here's a quick sketch to try to illustrate the setup, with the banner, pole, weight, trim tab, and harness:

Here, we're looking at about a 45 degree angle from ahead of the banner, with the trim tab adjusted to the left of the banner's direction of flight, causing the top of the banner to tilt to its right:

In other words, the tab at the top of the pole acts similarly to an airplane's rudder, where deflection to one side results in a force applied in the opposite direction.
More reading here:
Aviation Stack Exchange: How are banners kept upright?
Pilots of America: Banner aerodynamics
Quora: How are flying banners stabilized?