is nitrogen good or bad for the Earth?

Nitrogen is an ESSENTIAL element in the chemical fabric of proteins. So every herbivorous animal e.g. sheep, cows, horses have to consume enough nitrogen from within the grass that they eat in order to develop their body proteins.

Humans typically obtain their proteins by eating meat, dairy products but also from vegetable proteins such as peanuts and sweet corn

There are other plants rich in vegetable proteins in particular, CLOVER which obtains its nitrogen directly from the atmosphere. Peas and beans are also rich in protein. Plants may also obtain nitrogen from fertilizers which are commonly rich in nitrogen, phospherous and potassium.

Nitrogen may naturally enter soil in the form of nitrates as the lightening from thunderstorms results in atmospheric oxygen and nitrogen becoming chemically FUSED together and dissolving in the resultant storm water which is a very dilute nitric acid.

I that the nitrogen gas have 78% of the earth cycle.

It's good because many living beings need nitrogen. Animals can't use it in it's present form so they rely on plants and nitrogen-fixing bacteria.