You can be sued for anything. But you would win the suit. There is no copyright/trademark infringement because you are not using their name, likeness, or slogan. The only thing you are concerned about is defamation. However, your statement that “Dominoes Sucks” is not meant to be and could not be construed as a factual statement. It is clearly an opinion that cannot be subject to factual analysis. I’d say you are safe.
HOWEVER, while I love your spirit, others may not. Additionally, it does make people think you are competing with crappy pizzas when you are probably trying to differentiate yourself.
I have slogans on my shirt aimed at two local competitors (both of whom have a quality product). One is called “Waldo Pizza” and my shirts say “Where’s Waldo? Who Cares.”. The other place is called “Spin! Pizza” and our shirt says “Spin! This”. Neither say anything derogatory per se, but they take a dead aim at the places I want to topple. However, I have had one or more people take it the wrong way. I made the conscious decision not to recognize the cardboard pizza companies on my shirt even though there were some great slogans.