Mock ups, wax ups or smile designs are all part of the design phase in creating porcelain veneers.
Your question about who makes the mock up is especially insightful. At the level you are inquiring about, depends more on the actual duo of cosmetic dentist and lab technician. Many dentists do not do the design themselves. The majority are done by a lab technician.
The creative process really involves checks and balances at multiple different points in the creation of the veneers. An initial concept is created in the mind of the patient and the cosmetic dentist at the first meeting or consultation. After that a prescription is created by the dentist and sent to the lab. A good lab will add nuances to what the cosmetic dentist requested.
Then the veneers are prepared and the plastic temporaries are made. The plastic material has limitations in the level of finesse that can be created, so it is still an approximation of the original design and concept.
The original design is refined again as the patient gets used to the plastic temporaries. The patient may prefer certain changes and the cosmetic dentist may recommend some changes as well. All of these additional refinements are sent on to the lab.
When the ceramist makes the veneers he takes into account all of the refinements that have been made along the way. There are some final enhancements that really can only be created in the porcelain itself, so he builds them in then.
In the end, beautiful, fantastic veneers are created through great communication between the patient, the cosmetic dentist and the ceramist. ALL participants play irreplaceable roles. ALL must add their portion to the recipe in order to get amazing results.