If a cop gave me a summons without asking me to sign it, can i get out of it?

The signature at the bottom of a citation is just your written acknowledgment that you've been cited and that you promise to appear in court on the date listed. Because it was unsigned by you, it might be a defense if you don't show up on the court date, but it doesn't necessarily undo the underlying citation. By the way - I'm not suggesting not showing up on the court date. The "I didn't sign" argument might not get you anywhere with a traffic judge who's heard it all before.