Okay then, to clarify, ANY game can be modded if you're stubborn enough to keep at it. I was talking about clean-cut and safe modding that doesnt kill your computer every time you run it. :3
The thing is, when you mod games that have online components, things get crazy. If you modded a behaviour file, all things associated with it wouldnt be allowed to upload. If you modded a simple parameter in an editor, or a simple value for say.. how many resource units you get from a farm.. then everything made in the editor, and the farm, wouldnt be allowed to get uploaded. They wouldnt pass integrity checks, and if Spore cant upload decent-sized parts of your civ, it will likely just give up on your guys altogether.
Thing is though, mods can be fun and all, but I always avoid them if it has to do with a game that doesnt really support mods. Imagine downloading content, only to get an error message because you dont have the right expansion pack installed? Thats frustrating, right? Well mods take this to another level. Plus there is the problem that you can only really find out if a mod is reliable, when you install/run it. Mods in unsupported games have this notorious habit of messing things up, and given that there is an online component that will likely involve an accountname and password, there isnt a cat in hells chance i'll be running foreign programs with it.