There are plenty of actors whose skills do not extend to writing, and plenty of writers whose skills do not extend to acting, and plenty of actors who can write well and plenty of writers who can act well.
Suggesting that it is inappropriate to produce a script that is clearly sub-par merely because its author is an actor with some clout is perfectly fair. Suggesting that there's a particular correlation between being an actor and having sub-par writing skills is unfair and ignores the numerous writer-actors that have enriched our culture.
Including, I might add, that most famous actor who tried his hand at writing, a certain Mr. William Shakespeare of Stratford-Upon-Avon.