On the one side, it's a bad thing. On the other, it's stereotypical dudely behavior, and should be made fun of. "This is shit!" is, from my point of view, a way to observe that code 'feels' off, and an exclamation of frustration in that the reader can't understand it right away - and apparently doesn't have or can't take the time needed to dig in and understand it deeper.
I often go for gut feelings in judging code (this is shit!) too, so I can understand. I can understand the frustration too when one can't spend the time to understand and fix it - besides, someone else wrote it, someone else should fix it.
tl;dr, it's complicated. If someone exclaims "this is shit", go and investigate. Sit / pair with the person, walk through the code - or even better, if possible, find the person that wrote it - and either make it unshit, or make the person making the observation understand why it's not shit.