So, Dark Souls?
Jokes aside, if you’re talking about the sort of standard shambling unarmed zombies, vs guys in full plate… the only real danger would be getting completely overwhelmed by volume. Unless the Zombies can somehow bite through steel. Exposed leather joints, like on the gauntlets, may be vulnerable, depending on the specific armor designs. But, unless they can get through the armor, zombies aren’t going to be a serious threat.
On the down side, depending on how persistent the zombies are, the knights might not have any ready means of getting rid of them.
Also, if we’re talking about armed and armored zombies, then the Knights will lose eventually through attrition.