Most joke SCPs, obviously, are written for fun
But perhaps I have an idea that would make them part of the "official" SCP lore, without making them actual SCP objects (because really, how lame and unoriginal would that be)
Most of them actually exist, but don't have any supernatural properties (for example, SCP-628 is a replica of the portal gun but doesn't really create portals). Incoming personnel are assigned to these "joke" SCPs as a training exercise for the real ones. Those who perform to the Foundation's satisfaction become eligible to work on real SCPs; those who fail are released from the Foundation's service (often after having their memory of their time at the Foundation erased). Of course, given that Class-D personnel only get a month before being terminated, this may be impractical; perhaps Class-Ds are given a week on one of these trial SCPs, then those who pass go on to normal Class-D duty while those who fail are terminated?
Obviously, not all the joke SCPs can be worked like this. Other joke SCPs could be seen as entirely fabricated (as a security measure) or the result of some prankster within the organization.