It means "everything is perfect", perhaps in way that is "too good to be true".
I've not heard it before, but there are a number of other similar idioms:
- "everything is rainbows and butterflies"
- "everything is rainbows and unicorns"
It is quite common for English speakers to make up their own version of this idiom, keeping the format but replacing the nouns with other things associated with perfect, idealistic conditions - common substitutes being sunshine, lollipops, lemondrops and roses. Moonpies and salted peanuts are both snack foods, maybe considered "comfort" foods, and so are synonymous with 'feeling good'.
These expressions are just as likely to be used sarcastically as they are sincerely. In your example, it is being used sarcastically as it is being used to exaggerate and mock what the person on the phone is saying.