Well legal definitions are different in different places, but I would imagine it's usually something like indecent exposure or maybe sexual harassment. Not sexual assault.
My first instinct is yes, it is essentially flashing. On the other hand, I feel I would be way less likely to assign a sexual motive to mooning unless it was combined with some other sexual gesture or something.
Seems like it depends on who does it to whom. A 19 year old girl? She’s a whore. A 12 year old boy? He‘s a prick. A 12 year old girl? She’s been with the boys for too long. A regular dude/woman mid 30‘s?/40‘s? He’s/She‘s either mentally unstable or drunk. I do it to my girl? She’ll probably ask if she can finally put that thing from her bedside table inside me. An old man? He‘s a creep. A comedian on stage? It’s part of his show and he’s funny!!
Actually I don't think it does, at least depending on where you are, and the definition for sexual assault might be different, but in my state (US) you can be charged with assault even for intent to attack someone.