That's not really something specific to FSDT sceneries, but it's a general video card settings quality issue. Yes, you'll notice it more with sceneries that, for example, use custom ground and runway texture
It's likely you set video settings to default, which is not ideal for FSX.
The video setting that affect texture quality the most in FSX is the Anisotropic filtering. Set it to 8x or 16x, and be sure you set it to Override/Force the application setting. This is done in the Video card Control Panel.
In FSX, you can use the Addon Manager to set texture quality and HD textures. Our latest sceneries use 4096x4096 texture so, you must turn them on, in order to see the scenery with the resolution it was designed for.