Not that I hate this movie but it doesn't seem like the sort of movie that would get nominated for best screenplay. Places in the Heart was more deserving of that award.
"You want me to roll 6,000 of these!? What? Should I quit my job!?" George Costanza, Seinfeld
^
When you consider that most of the memorable lines were ad-libbed as well it's doubly surprising.
As for how an action comedy got a best original screenplay nod from the Academy an insider book about the Oscars I read some years back claimed that on the one hand there was alot of internal backlash towards the previous year's, 1984, crop of blockbusters (for the first time none of the top 10 top grossing movies of the year domestically were nominated for Best Picture). The nomination was a token bone the Academy threw to the year's most popular film.
I think it's a perfectly paced action-comedy. I've seen a lot of the genre... they usually have a lot of superfluous scenes. This one is nice and tight, and acted well.
That, to me, is what good writing is... I agree, it's not typically what the oscars go for...
Regarding the oscar nomination... I think they took into account Murphy's improvised lines as part of the screenplay. Murphy's improvisations are brilliant, he is a comic genius.