First of all, I don't perceive all those mns as being nns - or anyway as being the same kind of nn.
Leo,
Jules and
Liv are pretty standard as fns in their own right, and I wouldn't blink if I met a
Sophie who was just that, and not
Sophia. Which leaves
Katie!
So, let's focus on
Katie and, say,
Jimmy. If those were used as mns, I'd assume that it was for family or personal reasons - a wish to honour a
Katharine or a
James who always went by those nn forms. And that would be just fine; I'd like it. And I'd like the fact that mns tend not to be in daily use, as well.
Katie and
Jimmy as fns would get a very different reaction from me: regret that the parents were limiting their child's choice by giving a name that, by definition, did not have a nn form.
Katharine could be that, could be
Kate,
Katie,
Kathy, Ree,
Rina, and could change from one to another as fashions or her personal preferences changed.