Mon-el with the ring is overkill. Diana can win against a "standard" GL, and she can get wins against a Kryptonian/Daxomite, or similar (though in many cases she will lose the majority, if she isn't very careful), but she'd really have to play her cards right against a ring'd Mon-el. She can still get a win or two, via finesse, comparable physical power, and working the lasso/bracers for all they're worth, but Mon-el should take the solid majority here.
EDIT: this even hold true if the pics above are use as a premise -- she's looking/acting like a ho, and he's in total Pimp-Mode...
__________________ Why am I here commenting on this crap?
Seriously! She's either got a stone in her boot and sand in her ass, or she's Waiting for a tWenty and a trip to the VIP room. No Wonder everyone thinks Supes can beat your ass, you Whore! This is exactly Why Guy Gardner+ Dr. Fate slipped you that roofie at last years JLA/JSA party. Look at how you were dressed, you Were just asking for it!