it will not curl by nature This is the punch-line of Sir Toby's jest. Sir Andrew said that he wished he had devoted his time to "the arts" (languages, music, astronomy, etc.) In answer, Sir Toby said that if Sir Andrew had devoted his time to "arts," he would have "an excellent head of hair," which sounds totally illogical. But Sir Toby uses "arts" to mean "artifices" (such as artificially curled hair), so—since Sir Andrew's hair "will not curl by nature"—"arts" will indeed give him an "an excellent head of hair."