I didnt even recognize the jaw titan when I saw it in the last season of the anime. Why is this so? Is there any explanation in the manga or in any sourcebook why Ymir jaw titan and Galliard jaw titan look so different?
Galliard was probably injected with hardening ability, similar way Eren was injected with his hardening ability. And like Pablo mentioned, Ymir was very "Raw" titan. She didn't inject any abilities.
Also, two people possessing the same titan power can look very differently - for example, Grisha was super-hairy in his titan form while Eren has "elfen ears", and I don't think there's any specific reason for it.
I believe that it has something to do with the look of the titan holder, for example, Grisha's Attack titan had a beard, while Erens does not. Yamir has a very small jaw while Marcel or Porco has more pronounced jaws than Yamir.
My theory is that she was a pure titan for way too long so her body adapted to her pure titan form which is why her jaw titan looks like enhanced version of her pure titan. The reason I disagree with the hardening theory is because, The ancient jaw titan still had the headpiece and the hardening attachment marley use Is clearly different as is retractable whilst porcos/marcels is not