One of the most difficult things to fight against is cultural opinion. So I’ll begin by saying this: if your granddaughter is serious about her love for ghost-types, she’ll be fine. Despite their reputation, ghost-types are harmless when tamed; they’re simply mischievous and difficult to handle initially because of that. Your granddaughter may be aware of this, but if she isn’t, be sure to teach her to be clever and ever-alert in the presence of a ghost-type.
Wild ghosts, meanwhile, will let you know if they prefer not to be bothered. These are how violent hauntings come about—that is, hauntings that put humans in danger of physical harm. This is typically distinct from a mischievous haunting—the sort in which you may see objects floating about or in which ghosts pop up out of nowhere to scare you—in that when a violent haunting occurs, you may see anything from possession to ominous voices telling you to leave to a ghost physically attacking you (either through thrown objects or through an all-out battle). Often, even inexperienced ghost hunters are able to tell the difference; it’s when you grow to be an expert and thus a little overconfident that you let you guard down and come into harm’s way. Your granddaughter may already know this as well.
If your concern is more towards other people, I wouldn’t worry too much about it. Your granddaughter may already be aware of the stigma ghosts have in many cultures, even if ghost-type pokémon are nowhere near the malevolent forces human ghosts apparently are according to folklore. She may encounter resistance from the people she meets. Others may judge her or even bully her or her pokémon. But if she shows confidence in handling her pokémon and kindness towards her team, she may convince at least a few people that she and her pokémon are harmless. I would say, for your part, that you should support your granddaughter whenever possible, to show her that at least you believe in the best in her.
As for starters, for the most part, starter distribution centers will never offer a starter considered to be too difficult to handle for a new trainer. (There was one exception involving a boy from Kanto and a pikachu, I’m told, but even then, that was a rather unusual circumstance.) For that reason, your granddaughter will likely not be given anything like litwick or drifloon, but if she is, rest assured that so long as she knows how to handle them (that is, if she knows not to touch a litwick’s flame or a drifloon’s strings), then she should be fine.
Best of luck to your granddaughter, anonymous!