Have you forgotten Zoro's moment of character development? the fight against Kuma and then Zoro asking Mihawk to train him?
That's Zoro accepting that he wants Luffy to accomplish his goal of being Pirate King more than he wants to become the WSS. He started to care for Luffy and his crew, he has their safety above his own ambitions. Mihawk even congratulates Zoro for that, because he says now that Zoro has something to fight besides himself, he can trully become strong. So no, Zoro becoming a Guard Dog isn't regression or a bad thing, it his development and what he wants to become. The right hand man of the Pirate King first, then WSS.