"funny" is subjective so don't stress over coming up with something that would be entertaining to everyone. If you're stuck on what type of humor would work as a skit watch stuff like SNL or MADtv to get ideas on how to possibly structure a comedy skit and then apply relevance to your cosplay characters. Sometimes just observational humor works too because it's not anime or storyline specific that the general public would understand and find "funny".
Also, don't feel you NEED to write a funny skit for a masquerade. It's actually refreshing to see someone act out a serious scene rather then resorting to online memes, viral songs, or slapstick gags.
KATSUCON2014: JUDE-Tails of Xillia | YOSUKE-Persona 4 | KANAYA- Homestuck