What purpose might Jane Austen have had in making Mr. Collins such a ridiculous figure in Pride and Prejudice?

There are several possible reasons why Jane Austen might have made Collins such an absurd character. The first is simply that she is a comic writer and enjoys making fun of pompous, self-important people. 

A more important reason is his role in the plot structure of Pride and Prejudiceand how he serves to highlight the attitudes towards marriage in the book's social setting. In many ways, Collins is a very good match for one...

There are several possible reasons why Jane Austen might have made Collins such an absurd character. The first is simply that she is a comic writer and enjoys making fun of pompous, self-important people. 

A more important reason is his role in the plot structure of Pride and Prejudice and how he serves to highlight the attitudes towards marriage in the book's social setting. In many ways, Collins is a very good match for one of the Bennett daughters and most people in the period would have urged Elizabeth to accept him for that reason. 

By most basic ethical standards, Collins is a fundamentally decent character. Although he could have sought a more advantageous marriage, he feels some guilt over the entail, and does what is in his power to help the Bennett family by offering to marry one of their daughters. As he states:

 ...as I am, to inherit this estate after the death of your honoured father ..., I could not satisfy myself without resolving to choose a wife from among his daughters, that the loss to them might be as little as possible, when the melancholy event takes place ...

Even though we find his obsequiousness annoying and absurd, he is not an actively evil character such as Lady Catherine de Bourgh.

Thus when Elizabeth rejects his proposal, she is upholding an ideal that a good marriage is not, as her friend Charlotte Lucas thinks, merely a practical arrangement, but should be based on mutual respect and affection. Making him an unattractive character makes Elizabeth's choice seem more rational and less unconventional (from a 19th century perspective) than it would have been had Collins been a more tolerable companion for her.