The bug

Unlike many other blocks (composters, for example), hoppers do not have a dedicated texture for their bottom faces.

In 1.19.4 and earlier, the side texture was used on the bottom faces (MC-203399). This was changed to the inside texture in 23w14a due to aesthetic concerns, however this texture is still not unique and resource packs with a specifically designed inside texture could make hoppers look bad from underneath if not designed with this in mind.

How to fix

This should be a relatively easy fix - a new texture file could be created for the bottom faces and the hopper model files modified accordingly to apply this texture to the undersides of elements. In the vanilla resources, this could simply be a file which is a duplicate of the inside texture so that resource packs can modify it separately without having to change the model.