They allow them, but generally think it is a distraction from the design work. You can add your signature to the back of the card, which is what I do.  

I signed all my cards when I started at GCU MANY years ago and a year later stopped signing the front,  replaced the old cards with versions that had no signature and my sales began to sky rocket. When was the last time you saw a signature on the front of a card you purchased in a Hallmark store or from the greeting card aisle of a department store?
 
Fine art cards are signed and expected to be signed. GCU is not a place for fine art greeting cards. They are an any and all occasion online greeting card company. Few artists tend to have a pleasing signature - and - place it in a non-obtrusive location.
 
So, the choice is still yours, but if the reviewers feel it is distracting, you'll be asked to consider removal.  
 
Hope that helps!