Temperature is the measure of the average kinetic energy in the particles of a material. So, if you have a particle traveling really really fast, especially relative to a spacecraft also traveling really really fast, when they collide there’s a lot of energy. But nobody would call that temperature in the everyday sense.

So yeah if you look at the whole kuiper belt and measure the average kinetic energy of the particle flying around at high speed, it would have a high temperature, but that’s not necessarily a useful piece of information, unless when you say “what’s the temperature of the kuiper belt” you mean “how much kinetic energy do these particles have, because I would like to build my satellite shielding to not be smashed in by the first one it encounters”.