Is the heat of fusion endothermic?
Depends on the process.
Heat of fusion, or enthalpy of fusion, is misleading. It actually stands for both the freezing and melting processes, and as such, its sign depends on which process we're talking about.
As it requires input of energy to induce faster molecular motion, melting is endothermic.
As energy is extracted from a system to reduce molecular motion, freezing is exothermic.