So I imported a 3D model and the lighting with a directional light seems to be a little weird or off. Any solutions or ideas to fix this? 
It’s not the shader, it’s the shadow mapping technique Unity uses. When your directional light hits a surface at a very shallow angle, shadow banding like this occurs.
You can fiddle with shadow settings to control how shadows are generated, both via the directional light casting the shadows, and through the Edit → Project Settings → Quality menu.
Higher detail shadows, more blur, and smaller shadow distance generally help.