Yes, "c'est nul" is fine to express that, the other suggestions (at time of writing) are much, much milder than "this sucks". Other ways to express this sentiment are "c'est naze" and "ça craint". It can absolutely be conjugated, i.e. "je suis nul" and "t'es nul" mean "I suck" and "you suck" respectively.

That sucks

@AggieBlue Hello, Nul can have 3 meanings : - something very bad or with a poor quality : "sa chanson est vraiment nulle" (his song is really bad). - zero ; "le risque de neige est nul" (the risk of snow is zero/non-existent). - nobody : "nul n'est à l'abri" (nobody is in safe place). Also, in familiar language, we use it : - to describe someone, meaning that he is bad at something or acts badly or that his own entire person is like not interesting ("il est vraiment nul", he really sucks). - to describe something and say it's really really bad ("c'est vraiment nul", in the meaning : it is really a piece of shit). Have a good day.