I would guess that's something a fan artist did
(as a signature)... if a production artist or
voice actor signed it, the person would be sure
to sign their name even if they drew something
(I've seen signatures with cute little doodles
done by people involved in productions)... 

I can't imagine a collector would be crazy enough
to draw on a cel...

If you post the whole image, people can probably
help (through screen caps or whatever) to
determine if it is indeed a fan cel?

Peace!
Kim
aka Lady Z