Is there a sense of humour in Hamlet?

Absolutely. Almost every scene involving Polonius is very funny. In act 2, scene 1, for example, he sends a servant, Reynaldo, to follow his son, Laertes, to France and investigate Laertes's reputation by spreading a few rumors about him. Then, he should see if he can get anyone else to agree with him about Laertes and give up real information about him. In other words, Polonius will essentially damage his son's reputation a little in order to find out if his reputation is good! It's ridiculous.
Later, in act 2, scene 2, Polonius speaks to Claudius and Gertrude, and he is incredibly tedious and verbose. It takes forever for him to get to the point, and yet, he says that "brevity is the soul of wit . . . " (2.2.91) In other words, he actually advises others to speak briefly in order to be wise, and all the time he goes on and on, not realizing the irony. Gertrude, typically patient, even tries to hurry him along, telling him to speak more to the point.
Also in act 2, scene 2, Polonius and Hamlet talk, and Hamlet makes fun of him all the while, and Polonius seems completely oblivious. Hamlet greets Polonius as a fishmonger, and then says that he wishes Polonius were as honest as a seller of fish (essentially calling him dishonest). Polonius, however, doesn't realize Hamlet is messing with him or insulting him. Hamlet tells Polonius that he's reading a book about

old men [with] gray beards . . . their faces are wrinkled, their eyes purging thick amber and plum-tree gum, and . . . they have a plentiful lack of wit, together with most weak hams . . .  (2.2.196-200)

Polonius, again, doesn't realize that Hamlet is completely making fun of him, implying that he is such an old man with goopy eyes, no wisdom, and weak leg muscles. Instances such as these are funny because Polonius is a ridiculous and officious character, and Hamlet can joke about him without his realizing it.