"She has curly hair."
Translation:Mae gynni hi wallt cyrliog.
Yes, although then it would be 'gwallt'. The mutation comes after the 'gynni hi'
In South Wales the 'have' bit tends to come after:-
Mae gwallt cyrliog gyda hi.
In North Wales the 'have' bit tends to come before:-
Mae gynni hi wallt cyrliog.
All variations are acceptable
After:- Mae gwallt cyrliog (gyda/gynni/ganddi) hi
Before:- Mae (gyda/gynni/ganddi) hi wallt cyrliog.
The South Wales version of possession is easiest to learn since 'gyda' doesn't change with the pronoun.