Most Helpful Girl
It depends on the context.
People express their love/caring for other people in different ways. Often it depends on how you were raised or your culture.
My mom's side of the family is Danish and they kiss everyone on the lips (but a peck, not a "romantic" kiss). I'm closer with my dad's side of the family who does not do this (they express love through hugging instead). Since I don't see my mom's side of the family very often, it was always weird to have my grandpa or my aunt or someone give me a kiss on the lips, but it was completely normal for all of them.
If it's not common for this person to kiss family or other people who are close to them on the lips, or its the kind of kiss you'd give someone you had romantic feelings for, then I don't think it's appropriate.
However, you didn't say how old this god-daughter is. If she's an adult and consents to kissing her god-dad on the lips for whatever reason, then I don't see it as an issue. They're not blood-relatives (so it isn't really the same taboo as getting involved with someone who is a blood relative) and they're capable of making adult decisions about these kinds of things.2