Understanding the Meaning of "Makeup Applied with a Trowel"

The expression "her makeup was put on with a trowel" means that she was wearing a very thick and heavy layer of makeup.

A trowel is a tool used for spreading thick substances like plaster or cement. The image it evokes is of someone applying makeup in a similarly thick and clumsy manner, resulting in a look that is overdone, unnatural, and potentially unflattering.