I honestly think you're thinking of this film in the wrong context...
It's not a comedy...
It's a surrealistic crime caper with comedic elements, mostly because of their irony.
Many say this is the "funniest movie ever."
Many of those people are also trendy. There's always going to be those people. They hype it up as something it's not...and they probably also forget some of the lulls in the laughs while the plot advances.
I'd say it's a pretty funny movie, but not the best Coen brothers movie. It's got cult status, and the interactions between the Dude, Walter, and Donnie are really the crux of the humor. If you don't like the three of them and how they work together (or don't) then you/re probably not going to find much of the movie very funny at all.
The film is very dark...that is, it's a dark comedy. Dark comedies make us laugh at things we wouldn't normally. And those kind of jokes also have a way of not landing, depending on your sensibilities.
At the OP...could you imagine having watched this movie in the theater when it first came out...with no hype? Do you think your impression might be different? Many of the readers here at VGC are old enough to have been there...so it does have a special place in our hearts for being just a little different than all the other crap they release every year.