help with a rideable mob

mcreator can't do anything about that for now, if you want to ride lower you will need to code.

i suggest you submit a feature request in the Issue Tracker. but in the mean time if you're fine with coding, override the getMountedYOffset() method in your little ridable entity's class like this:

@Override
public double getMountedYOffset() {
    return 0.5D;
}
    

change the 0.5D to whatever height you want.

i suggest you submit a feature request in the Issue Tracker.

This feature is supported by 1.14.4+, issues for 1.12.2 should not be opened on the tracker. Rather pull request with a fix should be provided by the user in interest to the repository.

hmm, i tried to create a mob (2020.3/1.15.2) with the ridable boxes checked but i don't see where you can set the mounted Y offset. where is it?

It's on the first page of the entity, right next to the bounding box and shadow sizes.

You have to change the mob's hit box (entity model bounding box) on the first page of mob creator dialog.