Quite literally, someone who is happy doing what they're doing, or has just had a happy experience.
If you're not a happy camper, then you've just had a bad experience.
I went to sleep-away camp in the 1960's-1970's, and this was an old expression counselors used back then. They'd ask us kids, "Are you all happy campers?" to check with us that we were OK. It was a situational rhyme.