why does elizabeth ask mr.collins not to think of her as a lady the way he describes her?

Mr. Collins wants a lady convenience. It doesn't really matter which of the sisters he marries, in fact any of them will do.Elizabeth is an individual and is offended to be lumped into a broad catagory of identity.

Mr. Collins simply thinks she is trying to be a lady by upping the suspense, feigning that she is not attracted to him,

"My situation in life, my connections with the family of de Bourgh, and my relationship to your own, are circumstances highly in my favour; and you should take it into further consideration, that in spite of your manifold attractions, it is by no means certain that another offer of marriage may ever be made you. Your portion is unhappily so small that it will in all likelihood undo the effects of your loveliness and amiable qualifications. As I must therefore conclude that you are not serious in your rejection of me, I shall choose to attribute it to your wish of increasing my love by suspense, according to the usual practice of elegant females."

What a charmer!