As a matter of fact, I do agree with this criticism. Hamlet decides to act mad, and then waits for months before taking any other "action." Then, he seems only to make a plan once the traveling troupe of actors happens to show up at Elsinore. It surprises me that he would not be quicker to act, given the awful grief Hamlet feels upon the death of his father, the terrible way in which his father was murdered, and the fact that revenge means so much to to his father that his ghost returns to charge Hamlet with the responsibility of avenging him.
Also damning, for me, is the way Hamlet feels about himself. If he were making progress and pleased with that progress, I might feel less critical of him. Hamlet thinks he's not acting the way he should, though. Consider his speech after he watches an actor emote during a performance: he asks himself, "Am I a coward?": a seemingly rhetorical question whose answer is, evidently, yes (Act II, Scene 2, line 598). He also calls himself a
dull and muddy-mettled rascal, [who] peak[s]
Like John-a-dreams, unpregnant of my cause,
And can say nothing—no, not for a king
Upon whose property and most dear life
A damned defeat was made (Act II, Scene 2, lines 594-598).
Hamlet criticizes and characterizes himself as someone without courage who is, at best, an absent-minded dreamer who accomplishes nothing, despite the fact that he loved his dear father who was defeated in such a despicable way.