Yes nosebleeds can be a sign of cancer.
Especially if you are suddenly experiencing frequent nosebleeds those sudden and frequent nosebleeds can be a sign of cancer known as leukemia cancer.
So always get checked out to be sure especially if the nosebleeds continue and are really frequent.
Nosebleeds in most cases are not serious and can be caused by injury to the nose, dry air, cracked blood vessels in your nose etc.
Blood clotting is also another cause of nosebleeds but if the nosebleeds continue and are really frequent or you cannot seem to get the nosebleed to stop on it's own after a day or so then see a doctor to be sure it's nothing serious.