It is too thin. Handles are exposed so they dry first, heat first, cool first. There are shrinkages associated with drying and firing and expansion/contraction associated with thermal changes. Handles experience these ahead of the mug bodies. The heavier the mug and the thinner the handle the more likely cracking or shattering will occur. And handles need to resist the compressive and contractive forces of glazes that may not be fitted well. While specific stresses may not fracture a handle, they can be residual, seeking relief. That relief can come at a later stage in production (e.g. the glaze firing) or during use. It seems at times a miracle that handles actually stay on pieces and service for many years without breaking. What saves them is being thick enough to withstand the forces they are exposed to and thoughtful minimization of those forces during production.
In ceramics, handles need to be applied to many types of ware. There are things to know not just to make them look good, but to be strong, durable, practical.