Personally, I regard any signature or writing on cards as simply defacing the card. I wouldn't do it, and wouldn't want to buy one which had it, but everyone is different. I personally also think that grading cards is an astonishing waste of time and money (paying someone else to tell you the condition of something you already own, before sealing it in perspex which, in the case of many cards, especially Tri-logos, massively increases the chances of the figure punching through the blister and hence becoming worth a tenth of what it was before it was imprisoned in plastic), but again, everyone is different. I'm sure to some collectors having the 'right' signature on a MOC is a highly desirable thing, just as an amazing (to me) number of collectors send their MOCs away to the AFA or UKG to have them encased in plastic prisons.
It takes all sorts, and neither view is right or wrong, but to answer the original question, transparently a signature both does and should reduce a MOC's overall grade, even if that signature in some instances (Alec Guinness anyone?) might massively increase the value. If AFA/UKG grading is to mean anything at all, it has to be consistent. They are judging how close to a perfect example your particular MOC happens to be. Since it didn't leave the factory with a load of Biro or felt-tip scrawl all over it, of course the grade will be reduced because of it.
As for comics (which I freely admit, I know very little about), if the signature in question is the 'right' one, e.g. Jack Kirby or Bob Kane, then just like having the 'right' Star Wars signature on a MOC (Guinness, Ford, Lucas, etc.), then the value may very well be increased. However, I don't doubt there will be many comic collectors that prefer to have their comics without signatures, just as most Vintage Star Wars collectors seem to prefer their MOCs unsigned.
