I can cook but I don't consider myself a good cook.
Basically, I can follow recipes just fine, and eyeball some things (generally recipes that I've cooked often enough). However, I suck at grilling, rarely use the oven, and cant improvise. I think that understanding the science behind cooking (what physically happens when you heat something, what spices accent or compliment various dishes, using flour and starches to thicken sauce, etc) would be very beneficial. I've considered taking some classes, but for the time being I don't feel like I have the time or money to spare.
In the end, I'd say just about everyone can cook, since it's basically following directions. Being a good cook is more difficult and I think that label can only be applied to a small percentage of the population.