Stranger critiques various social institutions and mores. Actually, it critiques the way people don't question their purpose or usefulness. By critique, we don't mean it bashes them for all their flaws. That would be too easy. Instead, it questions aspects of human culture that many might consider normal simply because they are so used to their presence.
We get this critique from two major angles: Mike and Jubal. Since Mike is mentally a Martian, he views human society as an outsider looking in. Jubal, on the other hand, just comes by his critical nature through rigorous questioning and introspection. Which one strikes more of a chord in you as a reader?
But before you get fussy, remember that the novel rarely prescribes changes like a manifesto. It doesn't say stuff like X should be Y and Z needs to go entirely. Instead, the characters just suggest other ways to do things. The novel reaches some conclusions, sure, but it doesn't quite go as far as to suggest they are the best. The ever-feisty Jubal is a case in point:
"Public displays of rut I find distasteful—but this reflects my early indoctrination. A large part of mankind do not share my taste; the orgy has a very wide history. But 'shocking'? My dear sir, I am shocked only by that which offends me ethically." (33.23)
Notice how Jubal neither agrees nor disagrees that orgies (the "public displays of rut") are wrong. Instead, he considers both the way he was personally raised to view orgies alongside the ideas of others. Talk about open minded. And sure enough, Jubal's way of thinking sets the tone for the novel as a whole.
The critical and non-prescriptive are wrapped in a wry and dry sense of humor, the chaser that helps the medicine go down. Really, the humor is everywhere, you just have to be ready for it. So, are you ready for it?
"But 'As it was in the Beginning, is now and ever shall be' was so Martian in mood that it could be translated more easily than 'two plus two makes four' — which is not a truism on Mars" (8.76).
That's Heinlein's idea of a joke, and if you can imagine Stephen Fry or Hugh Laurie reading the audio book, you'll be in the zone. Heinlein's characters always have a quip or comeback at the ready, and they can go back and forth all day. It helps make the reading fun, rather than giving it the flavor of a science textbook.
Bottom line: read this book and laugh. After all, as we learn from Stranger, laughing is what makes us human.