“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.