I can think of two reasons why Mary’s opinion is not given. It plays no part in or reflects on other incidents in the novel and/or perhaps we are to draw her opinion from Chapter 22: She thought higher of Mr. Collins abilities than her sisters which is not much of a compliment since her sister’s think very lowly indeed of his abilities in the first place. She is also unsurprisingly struck with his solemn morality. Mary thinks she might be able to turn him into an agreeable companion if he will improve himself under her supervision but there is no declaration that she wishes to take on this tremendous task. The hopes that are done away with by the announcement of his engagement seems to have primarily been those of Mrs. Bennet and if any on Mary’s part existed they seem rather thin or perhaps held in check till Mr. Collins showed an inclination for her. Mary probably follows the same doctrine as Jane and Lizzy who prefer not to allow or reveal a preference till the gentleman has revealed his. The fact that Mary thinks Mr. Collins needs improvement says she felt he was not right for her as is and the idea that she would have to be “prevailed on to accept him” confirms she was not ready to capitulate if he made tentative attentions in her direction. If Mary had any real interest in Mr. Collins she might have been pleasantly agreeable to him after Lizzy’s rejection but she makes no such effort. Based on Chapter 22 I can only conclude Mary did not want Mr. Collins and was unhurt by his defection to Charlotte. ;D