Mmmmh... hard to tell. But I am gonna say no.Why? Because the first time they met each other, Mandark was still “Susan”, with pink clothes and long hair, and Dexter wasn’t already interested in him.I think, if it wasn’t for the name, Mandark would be mad at Dexter for pushing uh... her away for being a female. She would build her laboratory and try to be better than him just for that, because he laughed at her.I think she would try and make Dexter fall in love for her, and then use that to have him shut his laboratory down. But as we already saw, Dexter loves his laboratory more than anything else, so I think that, in the end, Mandark and him would have still become enemies.In all of this, I’m not sure were to place DeeDee. We could say Mandark could still have a gay crush on her, or she woud aspire to become her friend, and have a simple “girl crush” on her and would try to impress her anyway.
So, to answer the question: maybe Dexter would have a crush on Mandark at the beginning, but as she turned more jealous and evil, he would just completly ignore her until she tried to steal his stuff.