JJ,
I'll expand somewhat more on the suggestion I made here:
I suggest you not allow the subject to come up again by subtlety, very subtlety, showing a little more interest, contacting her a little more, and coming up with a few new ideas for things you both can do.
If she wears clothes/perfume
you like, puts her a hair in a style
you like, prepares a meal for you that
you enjoy, earnestly compliment her.
Send her a random text every now and then. Try not to say anything that could be misinterpreted. Playful texts are usually the best in this situation.
Plan dates at places and for events that both of you enjoy, but you especially. Don't be predictable. Take her to new places and do some different things, maybe even stuff you haven't done before but want to try.
The key here is to not make it obvious that you are doing any of this because of her comments, rather, you are doing it because YOU want to do it.
The fact is that you and her are BF/GF because you each appreciate the personality of the other, and as in just about every case, the personalities have some differences. You care a little bit less than she does. There is little you can do about that because that is the way you are. It is also very understandable because men often do not have the irrational thinking process women have when it comes to different things.
(JJ) said:
agreeing and amplifying relies on making the whole situation into one big joke. maybe there's another way to do it?
ABSOLUTELY DO NOT DO THIS. Making a joke, or even giving the appearance of this, is MUCH WORSE than "caring less". If your GF is really interested in this relationship continuing, she will not bring this up again unless:
1. She is giving you a sh*t test.
2. The original problem is continuing.
I reiterate, if you subtly make some changes, there should be no reason on her part to bring this up again. And you certainly won't bring this up for any reason. Based on what you stated the problem is, correcting it should end further discussion about it.