Being able to break down memory usage per mod would be a really useful feature for pack makers.

Thanks for the feature request. This is something I would love to add however I don't think it will be viable or realistic to add. There is no way to accurately determine the amount of memory used by a specific mod. Doing things like reporting the memory size of a specific object is also limited to JDK tools, meaning that the average user would not be able to make use of this.