Ymir's titan didn't even have the jaws.... Overall I am partial to the idea that isayama simply didn't do what he originally planned to do with ymir. At a minimum her role was substantially reduced. Here are my reasons for this:
Her titan's appearance simply doesn't add up. Both versions of her titan form are virtually identical. And while she did get claws, she simply didn't have the jaws that are characteristic of the jaws titan.
The way she was handled in the story. There are two important things to consider when it comes to ymir's role in the story:
The first is that her siding with reyner and berholt is colossally stupid and in no way justified by plot or circumstances. Shortly after she is revealed as a titan we see her siding with the two of them. Her excuse was something along with paradise not having a chance or something. And she makes the point that she could cut some sort of deal with the folk beyond the walls. But.... nothing we have seen since then actually justifies the idea that there was some sort of deal she could have gotten. Whether she succeeded in taking historia or not. If she had taken historia there... marley would have had no motivation to hurt historia. Historia would have been priceless. And even then, historia being there would not have given them any motivation to keep ymir. If anything, historia would have been the one to have pull over marley, her card being that she could agree to breed like a rabbit in return for ymir surviving. And even then marley had the option to give zero fucks about what historia and ymir wanted and simply kill ymir for her titan and force historia to breed. At a minimum it should have been obvious to ymir that her and historia's chances with marley were worst than if they stayed in paradise where ymir could have negotiated all of her information for some benefits. Ymir didn't act in any way that makes sense with what we know or what she appears to have known at the time. Isayama had ymir do a bunch of things which from her perspective should have been astonishingly stupid and hoped we wouldn't notice. And even towards the end... ymir even had the option of simply escaping reyner and berholt. Her titan is ideal for that. But she still willingly got on that boat to marley.
And the second part and where this gets really obvious... ymir's death. Ymir unceremoniously died off screen. She barely got a flashback telling us how little they got out of her. It's as if isayama just wanted to get rid of her. It's not impossible that we will see her again in some form of course, even resurrections seem straight up possible given how the coordinate appears to work. But still, after all the plot around ymir she turned out to be a nobody that was not even granted the dignity of an on screen death. Isayama got rid of ymir as if she was a flaming turd.
