In order to get a job in Nicaragua, you do have to be a resident or apply for a temporary resident card. You do have to have legal status in order to apply for a work visa.
The requirements to get a job in Nicaragua are no harder for a foreigner than for a Nicaraguan. You just have to make sure that you have legal immigration status in Nicaragua.
Perhaps the only job that a foreigner cannot take are the public positions, for example, the president of Nicaragua. Aside from that, I have never heard of foreigners needing a special type of permit. However, if you were a doctor and you would want to set up your own clinic, you might have to follow some procedures to legalize your doctor’s degree with the local administration such as accrediting your degree locally.