Portraits were definitely “improved” by artists to make the person look thinner, paler, etc. The happier the customer, the more likely they would get work within their elite inner circle (portraits were a luxury, not common for most of history).

They were done by skilled people with training though. Early paints were not easy to use as they were not nicely mixed in metal tubes. They had to be manually mixed pigments and binders (many containing lead and mercury).