i would say it has to be shameful because shameless s good.
What if a person would tell you that "You are a shameless person." How would you take it?
I believe both because shamless; without shame, could mean that a person never experiences shame regardless of what they do.
I've never seen shameless used with a positive connotation. It implies lack of conscience: you feel no shame, even though you should. Of course, it is often used in a joking context.
both contain negative aspects