"You're dumb" is disrespectful, but "your code is shit" isn't? How does the latter not reasonably imply the former?
Being respectful is taking the time to moderate "your code is shit" to something like "your code is not acceptable". You might even go a modicum further into kindness with "there are aspects of your code I need you to improve".
All express the same idea, some will leave the listener more open to internalizing the criticism.
Bad code, yes, calling it 'shit', no.
Stuff like this is a big part of why software circles are seen as so hostile and unwelcoming to outsiders.
You can be completely clear and frank without resorting to insult, mild though it may be. Just because you and people most like you understand that calling their work 'shit' doesn't reflect on them personally, doesn't mean it's not significantly exclusionary.
Now, obviously you can get to know your reports well enough to understand whom would take 'shit' well, but that doesn't mean it's not generally important to temper criticism with kindness. Kindness never has to mean holding back criticism, just avoiding stooping to insult.