The whole novel Of Mice and Men is filled with lonely characters. Candy is lonely as an old man with no family and no secure future. George is lonely for companionship because he takes care of Lennie and because he is a migrant worker and never has the time to cement friendships or relationships. Curley and his wife are alienated from each other, and Curley from the whole ranch it would seem. Crooks is physically lonely, as no one socializes with him, and there is no one else from his race or culture to socialize with.
Curley's wife and Crooks do not, by themselves, then, symbolize loneliness. Rather, the whole novel portrays the all-encompassing loneliness of a broken society in the depths of the Depression, and of those who are most vulnerable in it. This includes Curley's wife and Crooks, but it also includes almost everyone else in the story.
I do think they are both symbols of loneliness in one simple way: they are each a representative of an under-represented social class.
Curley's wife is the only woman in the entire novel who has a speaking part. Look at her name! She doesn't even receive an identity of her own. I think this shows that she represents an independent or individual or isolated experience.
Crooks, likewise, is the only black man on the ranch. He has to deal with being separated from the rest of the men because he smells different. He even physically has a different sleeping location. He too is isolated, representing loneliness.
Are you sure that there is supposed to be anything beyond the simple fact that they are really lonely and act really lonely? I do not really think they are supposed to symbolize anything -- at least not in the way that the farm that Lennie and George dream of symbolizes freedom or Candy's dog represents what happens to people who outlive their usefulness.
To me, Curley's wife and Crooks just ARE lonely, as you say. They show how loneliness affects people -- Crooks tries to destroy Lennie's dream even though he really wants to share in it, Curley's wife acts in ways that will get the men in trouble even though she really wants their company. But I do not see them as symbols. I see them more as characters who are defined by their loneliness.