Jack's interesting Servant design are the concepts of a collection of souls summoned as a single being and Jack the Ripper having many possibilities due to being unadentified. How are those positive additions to the story?
If we take the JoJo approach as you named it, she's wholly underutilized. Being a collection of souls has no effect on her fighting abilities nor does it add any complications to fighting her. Jack being undefined also doesn't do anything on that front because identities here are only relevant if you have a literal weakness. What else is there? Her most interesting skill is Information Erasure, but there's no extended crime solving section for that to be used effectively, and it doesn't complicate the final fight in any way either. It's immaterial beyond a short, innefective encounter with Chiron. There's also her Noble Phantasm, which is supposed to be an easy win against women but is simply countered by Jeanne being Jeanne. If we were judging her like we judge powers in other battle-focused series, this would be an awful waste of something that should bring a lot of tension in a fight whose main powerhouse is a woman, but that never materializes.
Narratively I don't even think I have to comment. Jack is an aside, a sidequest that has no importance to the story. Her nature as an amalgam is the biggest contribution she has and it's only to force Atalante into conflict with Jeanne, which she already was, but this time without any chance to voice a viewpoint more complex than dying children being sad. I can't say I appreciate having one of the characters involved in the final conflicts of the story completely removed from whatever nuance they could have had.
Thematically she fits, I'll give you that, but once again I don't think any of the merit is in her Servant design. In a "story about wishes" she's pretty much just the early representation that the wishes of the dead shouldn't interfere with the living. She's there to prep for Amakusa but... For what purpose? Neither Jeanne nor Sieg change their stances in any significant way after meeting her, their response to Amakusa's wish would be the same with or without her presence and, even if it did, is that in any way connected to her Servant self? Her tied to her backstory sure, and the fact she's a bunch of wraiths is part of it, but in what interesting way is it used? She could be a single dead kid and the result would be the same.