Inline Entity Form doesn't support Comment forms out of the box.
Part of the process of building the IEF widget is a call to
inline_entity_form_get_controller() which will retrieve a controller class from the entity metadata. As you can see in IEF's
hook_entity_info_alter() invocation, it only adds this controller metadata for node, taxonomy, commerce product, & commerce line item entities.
Theoretically, you could build your own custom module that could:
hook_entity_info_alter() to add support for comment entities.
- Build your own custom class that extends
CommentInlineEntityFormController) to be evoked for form processing.
Keep in mind this is an unusual use case, a comment entity is predicated on a its parent entity pre-existing before it's created; you're likely to run against logic that might prevent this (e.g. database integrity checks) .
It might be easier just to create your own custom node type entity that's configured just to appear like comments on your nodes.