It seems made up. That's what I dislike about it. People will always think you were creative and there are so many -issa names out there, it reminds me of the -ayden trend.
Still I kind of like it. I also like
Elissa but wouldn't use it. People would always assume I made it up or that I butchered
Alyssa even though it's a separate name.
I don't have anything against made up names but I have the feeling that
Carissa will be perceived as trashy by many. I think prefer
Charissa or
Charis.
But it's also kind of wrong because in Greek the Ch in
Charis is pronounced like an H. I guess it could still be a variant of
Carys which is pronounced similar to
Carissa.
Bottom line: I like it but I wouldn't use it for the reasons mentioned. It would make a good middle name, though.