Firstly, I know there is a growing voice against this type of play as they claim it perpetuates pedophilia. I find this to be patently false. In my experience, it's not pretending to be someone's actual father. It's related to dom/sub play: daddy being the authoritative figure would lend itself to the dom position. One of my former partners turned me onto it, calling me daddy out of the blue as a way of letting me know that she was enjoying it so much, that she felt like I was owning the sex and not necessarily her. I've been enjoying it since, and have expanded it to daddy/daughter role play in person or online, but never once fantasizing about actual members of my family.
Secondly, I would ask her about what it is that she finds exciting (without judgment) and basing your role on that. If it's just something the wants to call you in bed as a matter of letting you know her current enjoyment, go with it. If she wants to be dominated, I would discuss with her what else she desires as a submissive, make an agreed set of rules, and of course, agree upon a safe word. If it's actual daddy/daughter fantasy without it being about a real family member, ask if it involves age play, what scenarios she might like to try, costuming, etc.
Third, above all, ask yourself how comfortable you are regardless of her reasons. I don't personally believe in the stigma that it is bad. However, if I were a person was traumatized by sexual abuse from a parent, sibling, or other family member, I may feel differently.