Can a short circuit in the output of a transformer be dangerous?
If you short the output of a good/efficient transformer, the secondary current will be very high, and the transformer will likely overheat and be damaged.

However, there are intentionally lossy ("impedance protected") transformers that will limit the load current to a safe value to prevent damage to the transformer in case of short circuits or overloads (often used to power doorbells and similar applications).