When I saw Ken do a category, his delivery was what I thought was too slow for the pace or Jeopardy!.
One category is nowhere near enough to judge someone's hosting abilities. Finding the proper pacing and way of melding your personal style into the format is something that takes a few episodes at the very least. [ETA: Also, having looked up the relevant video on YouTube, he was standing in front of a green-screen in a series of prerecorded videos for those. They're not going to reflect a more natural delivery inherent to "doing it live", so to speak.]
Hell, it took Drew Carey two
seasons (or more, according to your taste) to shed the new-host awkwardness when he took over
The Price Is Right. Part of that is because hosting TPIR is a few orders of magnitude more complex than
Jeopardy! (and requires instant-recall of the rules of 75 different mini-games, some of which show up once every few months or so, whereas J! is the same thing every day), but I don't think judging someone's performance by one
category is remotely close to fair.