I mean mostly it's a kind of truth taken to the extreme I guess.. I'm afraid it won't be very helpful or even funny to a non-swede :p Wonder if it's the same with the Japanese one.
But my understanding is that this is the contrastive は, so there is somewhat of an implication that there is some unimportant/trivial stuff, IMO.
Or, well, I guess maybe not.
It depends on the pitch.
大したことは[ないと思います], with は just being at a normal pitch would be just a normal topic marker, while [大したことはない]と思います with は being at a raised pitch would be contrastive