Maxperson
Morkus from Orkus
In some cases yes, others no; or not so much.
Skeletons, lycanthropes and the like can be beaten without knowing their weakness / vulnerability. It just takes longer, but the challenge doesn't rely on it as they can be dealt with in other ways.
Trolls, vampires, and other creatures that can only be killed by knowing their weakness, however, become a LOT more challenging if you don't know (or learn during the encounter) their particular weakness / vulnerability. If you don't know it takes fire or acid to put a troll down and keep it there it Just. Keeps. Coming. Back. If you don't know how to finish off a vampire you're most likely gonna see it again once it's had a little rest in its coffin. And so on. So here, knowing or not knowing the weakness makes a huge difference to the challenge presented; the challenge itself doesn't rely on the weakness but the ability to overcome it does.
Lanefan
There's also a lot of fun to be had in winning, but not being able to finish the creature, or losing and escaping, then setting off to find that information so you can come back and defeat the foe.