Yes, the shirt also has to have a morph target. However, the shirt won’t morph unless it’s the Inherited Mesh. Chances are, it won’t be. Chances are that your character’s body is set as the Inherited Mesh. The bug will not morph the shirt with the body. What is supposed to happen is provided the name of the morph is the same on both models, both models will morph at the same time. For instance, if you have a morph target called hipwidth on the shirt and you have a morph target also called hipwidth on the body model then you only need to send one Set Morph Target call to the Inherited Mesh. It would not only morph the body but also the shirt because both models have a morph target with that name. Only problem is, all versions beyond 4.8.3 are broken in this aspect and will only morph the object assigned as the Inherited Mesh (the bug of doom.)
Okay so I understand that, what is the best way to apply morphs to the T-shirt also so that they morph perfectly. The method I am thinking of will require me to write a plugin for blender as you can’t just copy a morph to a new object mesh unless they have the same vertex count