It's not a viable heat sink because it's a thermal insulator that doesn't support transport of heat. The thermal conductivity of lunar regolith is lower than rock-wool insulation,

https://pmc.ncbi.nlm.nih.gov/articles/PMC9646997/ ("Thermophysical properties of the regolith on the lunar far side revealed by the in situ temperature probing of the Chang’E-4 mission" (2022))

https://www.engineeringtoolbox.com/thermal-conductivity-d_42...

(Imagine, for entertainment purposes, what would happen if you wrapped a running server rack in a giant ball of rock-wool insulation, 50 meters in radius).

Only way to dissipate large amounts of heat on the moon is with sky-facing radiators.