It's probably safer than kissing most humans. Fewer diseases cross easily between species than between members of the same species.
You're probably not washing your hands every time you pet the cat, so you're already exposed. If the cat is sleeping on your bed that's additional exposure. You're getting some exposure from the litterbox as well, of course.
Most of the illnesses the cat might have would be ones you brought into the household.
A cat does have some exposures you don't, from sniffing at your shoes for example, of from hunting/eating small critters, or simply from the fact that they bite/lick/mouthe more things than we do. And being indoors is not complete protection against biting insects, which can carry disease or parasites.
But my own conclusion has been that I accepted some minor risks when I decided to bring cats into the household. Having made that decision, I'm not going to worry a lot about it. The cats and I both get a yearly checkup, plus a vet visit when we seem to be ill, and that's good enough for me. (Actually, the cats see their doctor more often than I see mine.)