Could you explain, Merry Andrew, what you mean by 'in-group' jokes? I'm sure that his work isn't for everyone. And what do you mean by calling yourself an outsider? Does that make fans of his insiders? Please explain.
Oh, I've read a lot of Heinlein and liked most of it. But his humor very often presupposes that you've also read some of the same things he's read (not always standard stuff that just everybody should know) and will therefore 'get' the sly allusion. For example, he makes references to John Carter of Mars, the Edgar Rice Burroughs character. I get it, but I also think, "That's probably not funny for most people who've never heard of Burroughs apart from the Tarzan books."
By and large, Heinlein does not use very much humor in his writings. (I should say 'didn't use'; he died a couple of years ago.) Stop me now, as this isn't a Heinlein thread. Anybody wants to start one, I'll be happy to participate.
I suppose you are right. To tell you the truth, I feel kind of warm and fuzzy inside when I catch one of those obscure references.
some of the best humour is self deprecating comments;
especially effective if it puts down the strong points of the comedian, therefore adding disbelief to the 'surprize' reaction.
(i mention this to diferentiate from 'mean' humour based upon comments about others)
Right, Bo. Think Rodeny Dangerfield and 'I don't get no respect.'
Yeh, but neither did he deserve any!
[how about your signature line!

]
BoGoWo, self-deprecating humour is technically masochistic, not sadistic, so I think you have answered the original question.
Are all internet posts silly?
Everything is 'silly' from one point of view, or another!