Yes there is an acceptable target.
Is it amoral, yes. Is it counterproductive, probably. But people will say whatever they want whenever. It just speaks to large issues in society like poor health/mental care, trauma support, and poor education. Also, it depends entirely on the body. Let me explain.
Someone that is disabled, just looks different, or something else physical out of their control shouldn’t be shamed for it. Someone that is overweight also shouldn’t be shamed but should be able to acknowledge that it is(probably) not the healthiest and get adequate support. Same with unregulated steroid or narcotic use.
Albeit not the best way, it can be beneficial to maintain these stigmas against certain groups in society if it even slightly reduces the number of people who chose that path.
As things stand now stigmas against appearances that are harmful to the body shouldn’t go away. I think these are the “acceptable” target for body shaming.