The concept that gets repeated all the time nowadays is to
prepare situations, not plots, and I think it's good advice.
The party can't really encounter
any monster. If they're in the desert, even if they go off map, they're probably not going to be dealing with a yeti or a sea serpent. If you want to come up with a few extra monsters that they might encounter in adjacent hexes, go ahead and stat up maybe the most likely monster in each.
And remember,
you can always just take the stats of a bear and reskin it on the fly. Take off its fur and skin and maybe have green glowing goo dripping from its mouth and eye sockets. Since it's DCC, it can be a one-off monster that's never explained or named, and the players will have a good time anyway.
I like to have a list of NPC names I make ahead of time, using online generators or translation tools and then, when the players decide to befriend the stable boy for some insane reason, I've got a name ready to go.
Roll & Play makes an
amazing book to help gamemasters on the fly, the
Game Master's Fantasy Toolkit (they also have a
sci-fi one), which includes tables for all sorts of stuff, like the name of the tavern, what's in that pocket that just got picked, the effects of critical hits, what the bard is singing about, NPC jobs, etc., all in an easy to use spiral-bound format. (Big books of tables that are hundreds of pages long that take 20 minutes to find the table you want defeat their purpose, IMO.)
For my games, I know what the base situation of the setting is, what various NPCs' goals are and what they're doing to accomplish them and what the inciting incident for the players is. From there, I am prepared for my players to throw whatever they want at me. (And I have some players who love to throw curveballs.) I figured out most of the above from trial and error, but it works very well for me now.
You've got this.