Answer :
When the author claims that Helen Grey miss that modest charm, which is the surest charm of all. He might be handsome of all, but when he will trip and fall, no man would care to stretch his arm and help. This shows that Helen is an unpleasant person.
The passage "Because you're handsome, Helen Grey, Is that a reason to be proud? Your eyes are bold, your laugh is loud, Your steps go mincing on their way; But so you miss that modest charm Which is the surest charm of all: Take heed, you yet may trip and fall, And no man care to stretch his arm." explains that Helen Grey isn't a very pleasant person. In the first part it's saying that Helen Grey is very proud. Then it goes on to explain how Helen Grey acts like she's better than everyone because of her charm. Finally the narrator tells Helen Grey that because of her pride and her unpleasantness, she needs to be careful because when she needs something no-one will want to help because of the way she acts.