Someone who is overconfident, outspoken, histrionic yet possess few redeeming qualities (cf. salad face 1a). A moon person is typified by their penchant for sneaking into conversations that do not involve them in any way, and speaking with false authority. Frequently stories they tell are fabricated on the spot, usually unconsciously.
She's a real moon person. Every time I tell a story she has to one up me. She's squatted 500 pounds and been to the moon too!