Charlotte says not only that she is not romantic but that she never was. So in answer to the question in your subject line, I think that the answer is "neither." Charlotte is what she is, and she has been that way for a long time. It is not a matter of either maturing or giving up hope. If Mr. Collins had some along five years ago and been rejected by Lizzy, I think Charlotte would have reacted just as she did at 27.

I disagree that one never explains one's reasons for marrying. If a close friend had said to me, "Engaged to Mr. Kitao! my dear Kathi, impossible!" when I informed them of my engagement, I might have felt the need to explain, not because I had any doubts about my choice but because I wanted this close friend to understand.

Are you suggesting by questioning Charlotte's choice to explain her decision to Lizzy that Charlotte herself is not sure about the decision herself? If so, I think the fact that the narrator (not Charlotte) tells us that "she felt all the good luck of [her engagement]" suggests that that was not the case. Charlotte was quite satisfied with her decision.