A useful CRT shader would be quite a complex feature, as decent CRT emulation is not as simple as slapping a texture on top of the artwork. On top of needing a UI to handle all the parameters it needs, it would need to generate shadow masks, curvature math, and colour remapping on the fly according to the parameters (avoiding the complexity of that work is likely why SuperCRT has a fixed screen size that it targets), possibly with its own set of presets.
This seems like a feature best suited to either an external program like SuperCRT, or, more conveniently, a plug-in that adds an additional preview window to Aseprite. I don’t think this should be a core feature, as it’s a very large amount of work (and eventual file size) for something that is at best a curiosity for most users.