Thank you for reaching out, currently the thumbnail will be generated based on the angle you are viewing the object from, and the object will appear the same as if you were to press F or use Zoom to Selection. Adding custom materials to the User Library should display very similarly to materials from Twinmotion's Library. If you would like for this to be more customizable, please use the +Submit Idea button on our Twinmotion Public Roadmap to provide this feedback to our development team.
In the meantime, it may be helpful to experiment with isolating the object, moving your camera in the viewport, and using the F key to focus on the selection, as this will be a preview of the how the object will look when it is added to the user library.
In 2023.1 Preview 2, the size of the interface can also be adjusted through Preferences > Appearance, and you can increase the Interface scale to better view the library thumbnails.