What is the ruling on a girl who rejects suitors for reasons such as age, appearance, or a lack of comfort, and is she considered sinful for that?

There is no harm in rejecting a suitor who is unsuitable financially, educationally, or in age. Indeed, considering these matters is good. Rejecting proposals before seeing them is not sinful, but excessive rejection and exaggeration in conditions can have undesirable consequences. One must beware of showing discontent with divine decree and instead be content with it, for everything that happens is by Allah's decree, wisdom, and mercy. Allah the Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not." Have good expectations of your Lord, rely upon Him, and supplicate frequently.