Exactly, just because Wally is an ignorant doofus in need of a beating doesn't mean that Alice wants to work in an office with coffee-stained carpeting and shards of porcelain all over the floor.
Plus, placing the coffee cup on Wally's back provides a little more emphasis on how easily and thoroughly she owned his pasty butt.
saywhatwhat is right. Alice might even clean up the spill unasked. There is a strip where Wally teaches Asok the power of "low standards" by not returning the dishes to the cafetaria. Then Alice walks by and is compelled to clean up the mess. See the strip of 24-12-2006
Ok, I never did talk to Alice about her motives but, "... she wouldn't be forced to clean up coffee spills and, therefore, wouldn't be bothered by the possibility of being asked to do so". is a classic failure of logic. More than one in fact. Being asked and being forced is not the same thing, and even if it were, she could be "bothered by the possibility" without it being possible that it would in fact happen.
And as for the possibility that Wally put the cup there, it's obvious that his arm is too short. It's not like this is a cartoon... :-)
If Alice were as epic as everyone made her out to be, she wouldn't be forced to clean up coffee spills and, therefore, wouldn't be bothered by the possibility of being asked to do so. Also, it's more amusing to think that Wally saved it. VICTORY!