Thomas Hill is a power plant lake - the water from the lake is used for cooling of the power plant. That tends to make the water warmer. So while it is true that northern lakes are cooler, Thomas Hill is kind of an exception, as it has an artificial warming mechanism built in. It is actually popular when it is cold outside, as the warmth actually slows it down when it comes to freezing over.
That said - there are definitely crappie up there, but my experience is that they run on the small side. Crappie seem to go in cycles with certain years being better for the big fish then others - but my experience on Thomas Hill is that the crappie there don't go through that cycle - they are consistently small...