You have to consider how "Sudden" the crushing is inteded by the artist, if it's a quick surprise crush then the yukkuri won't be able to hold at least it's puke, while a slow crushing will give it time to react and try to at least hold the puke.
Also the angle is fine, considering this is a small yu that means it's easy to crush, remember yu's are soft, if it were a proper adult yu then it would be another story since the size would need a more "from the top stomp" crushing angle.
There's a thin gap between marisha's body and shoe. Also, area of contact are too small, if this is a 'sudden' scene that would mean violent impact where as the depictions lack 1) sign of resulting deformation of the target 2) 'move lines' required for sign of speed if the shoe was indeed intended for sudden movement.
The way i see it is marisha's body is the result of an off-screen prior action ---> the body swelled back with the traces of vomit and poo. But the vomit and poo were in the process of being pressured out though, so ... if only they were drawn as 'dribbling out' and;
the shoe was removed or shown to be in position of 'anon standing nearby' the scene would be more believable.
By sudden I mean out of nowhere and without Marisa realizing it. Plus, considering the size of the yu, you're givint too much credit to the resilence of yu's, a normal stomp and already end a full grown adult yu, so this ko sized yu would be easy to crush with a half step.