It really is going to depend when you are going. . if you're in late Jan, then they probably are already ramping up for Mardi Gras (it ran for multiple Months last year), so that might draw in bigger crowds for the special food offerings/parades/shows, etc.
keep in mind, in Jan, the waterpark section (Whitewater Bay) will *not* be open. Honestly, the best time to visit Fiesta is in the hot Texas summers, as the waterpark will drain a LOT of the crowd from everywhere else in the park until late afternoon - leaving really empty ride queues for most of the morning/early afternoon.
Can't say how it's gonna be when you visit, tho I wouldn't pre-purchase. I'd wait until you see how the crowds are first.
note: at the moment, Wonder Woman is NOT on the Flash Pass (it might be by the time you visit, but for now, it's not).
so the real benefit you'd get would be for Joker (giant pendulum), Iron Rattler, Poltergeist (Flash Pass drops you directly in the station), and possibly Superman (if the park is packed).
it's also good for cutting lines on Bugs Whitewater Rapids (flume) & Gully Washer (Rapids) - tho if it's cooler, which it will be in Jan, they might not be operating (or might have no lines regardless).
and *maybe* they'll open the new ride in Jan. . it's ready to go, I could see them opening it for Mardi Gras Celebration.
the Flash Pass office at Fiesta is to the Right of Boomerang - which you dead end into when you enter the park, and keep walking straight ahead.