It has been a long time since I perused D3 (I played through it once too, but I don't remember the daemons coming up), and I don't have a copy, but it might be interesting for the discussion if anyone knows off the top of their head what the daemons are doing in that adventure and what their business is in the Underdark. It might shed some light on why Gygax, et al. felt it was desirable to introduce a third group of fiends into the game.
I think Shroomy is right. I don't know what Gygax had in mind, but as presented in D3 mezzodaemons and nycadaemons are not distinguishable from demons: they're evil beings from other planes that the drow seem to enjoy hanging out with.They're muscle and guards. Nothing more grandiose!
In Gygax's DMG, they appear in the glossary, but are described simply as "monsters" (the glossary entry explains that you can ignore them if you don't have D3); in the descriptions for Iron Flasks; in the rules for holy water and for turning undead; in the rules for aerial combat (nycadaemons only); and in the rules for random encounters (dungeons and cities; in the latter case they are alternatives to devil and demon encounters). The closest thing to any flavour text for them in the DMG is the description (on p 51) of how nycadaemons
act much like flying battering rams, striking for 2-12 points of damage when they hit (or as much as 3-18 if they hit another flyer head-on, though this will also damage the nycadaemon 1-4 points) [who will] try to run . . . prey close to the ground and then close for a grapple. The wings will be used to batter and confuse and slow their fall, as [a nycadaemon] attempts to bring its opponent down to the ground where leverage and its full physical strength can be brought to bear.
Type IV and Type VI demons are said (on the same page) to "try to bring their foes to the ground, much like nycadaemons" - so this closest thing to flavour text in the DMG does not identify anything distinctive about them!