Ridiculous has as much a negative connotation as silly does. There are good silly things (that's a silly little song I love) and bad ridiculous things (it's ridiculous that they made us wait in that 100 degree room for 5 hours, someone almost died!).
The text does explain how to tell someone one's opinion on what they just said though. That's pretty much what using "ridiculous" as an exclamation is doing. Without any more context from qunnoq I don't know if he wants to use it as an adjective or in as an exclamation like this:
I ate 20 pies!
That's ridiculous/stupid/silly/nonsense/etc
All those can be covered by {Dap} and/or {chatlh}.
qurgh