You can edit an airport background polygon to blend the airport into the mesh with Airport Design Editor - if you have the Deluxe version of FSX and have installed the SDK.
Or you can use SBuilderX.
The problem created with FSGenesis is two fold. The default airport is not at the correct elevation - so when FSGenesis, or any realistic mesh, puts the area around the airport at the correct elevation - the airport stand out. The second is the FSG mesh is more detailed than the default mesh so the imperfections of FS show up so much more. Sloped airports are a real problem.
KCOS and KLAS are two of these.
KCOS airport elevation is 6187 feet which in the real world is at the north end of Rwy 17L/35R. The south end of Rwy 17R/35L is elevation 6045 feet in the real world.
That is a difference of 132 feet.
The FS airport has to be flat - so the south end of Rwy 17R/35L is 132 feet above the underlying terrain mesh. The default mesh takes about 400-600 latera; feet to make the elevation change. When you add a much more detailed mesh like FSG - the 132 foot elevation change occurs in 20 lateral feet. So what is an unrealistic hill in default mesh becomes a 132 foot cliff with detailed mesh.
KLAS is similar with a 103 foot elevation change between the south end of Rwy 1L/19R and the north end of Rwy 1R/19L. The far east end of Rwy 7L/25R is 148 feet lower than the airport elevation.
Lakes are another matter because water is not subject to terrain mesh, but takes a different type of polygon to change.
@ PawPaw's house - near KADS, Addison, Texas, USA