Very lovely!  Congratulations! ^_^
I don't collect DB/Z cels myself (only got a 
couple) but my roommate's really into it, and 
most of his cels have similar fading.  I couldn't 
speak for all cels, but the DBZ cels I've seen 
for sale/on display at the last couple cons have 
mostly had a fair bit of fade, as well.  I 
suppose it's just the age, and the studio's 
storage practices (which I've heard are not so 
great.)  As such, I wouldn't think it's too big a 
deal.  Especially not when the cel's so great. ^-^
Really adorable shot. ;)