Complex sentence: It made him one of, if not "the" most popular painter in Amsterdam, and he was frequently hired to decorate the interiors of government buildings and homes of wealthy businessmen with lavish trompe l'oeil ceiling and wall paintings.
Split into simpler sentences: It made him one of, if not "the" most popular painter in Amsterdam. He was frequently hired to adorn the interiors of government buildings and homes of wealthy Amsterdam businessmen with lavish trompe l'oeil ceiling and wall paintings.