Very slim. Chef Mickey's is usually booked solid. If you want to try, you need to show up a bit before they open. They may be able to fit you in. Or if you can, call the day of and check for an opening. But walking up to lunch or dinner is almost impossible.
We got a walkin over Christams/new year last. Had been there and kids were begging to go back! As PP said we arrived about 20 mins before opening and were seated within 45 mins. However we were only 4.
My cousin walked up and got a reservation for the same night we were there in August for her party of 5. Never happened for me but she says she does it all the time.