Ha! The really tough ones! :-)
I headcanon the Rohirrim as being fairly literal-minded people. Of course they would have metaphors and word-play as poetic devices; but their humour is rather more …earthy. Add to that the fact that Westron is their second language. So by and large, puns, innuendo and sarcasm would most likely earn you a blank stare.
Now, Helmie is an educated girl, she has had some contact with other cultures; so it’s hard to say if innuendo is totally wasted on her, or if she is just very, very deadpan.
She’s a guarded girl, but she will laugh readily when she’s relaxed and in the company of people she feels comfortable with. She delights in the new, the surprising and the unexpected. And since this is a warrior culture, she’s been known to laugh in combat situations out of sheer grim joyous fatalism.
Of course, she has her moments of silliness. For some reason, goats that sound like people make her giggle uncontrollably.