Why are some of my preset names are in italics?

No, it's not about Lightroom Classic versus Lightroom cloud. Partially compatible presets are normally not presets that were created in Lightroom Classic and so could contain features not available in Lightroom Mobile, but presets that apply a raw profile. If the selected image is not a raw image, then such a preset is not compatible and shown in italics. Another possibility is that the preset applies a camera-specific profile, such as "Camera Landscape". That creates a preset for one specific camera, so it is incompatible with raw files of another camera.