Ferrite Magnet as Ferrite Inductor Core?
Choosing the right ferrite for a particular job has to be done with care and take account of: -

the cores basic magnetic permeability
the cores losses vs frequency
the cores permeability change with temperature


Could I make an inductor with one of these + magnet wire and if so,
just how crap of an inductor would it be?

Yes you can make an inductor with it but, regarding how good it will be is really impossible to say. Why bother when there are tons to choose from and all of them should have data sheets.