The supposed similarity between Mara and Illya has been forwarded in several slash stories, but honestly I don't think the writer(s) saw that at all.

The idea of the "bookish" blond turning into a femme-fatale was not a new idea, and I think that was all that was intended here with Mara.

Plus the Illya of the series is never really the "bookish" sort that is often portrayed in fan fiction. That type of characterization for him is very heavily "fanon".