Node 18's fetch function isn't built on the Node.js http module like most HTTP libraries (including Axios, request etc) - it's a total rewrite of an HTTP client built on the lower-level "net" library called undici . As such, "nock" cannot intercept requests made from the fetch function (I believe the team are looking to fix this, but at the time of writing, Nock 13.2.9, Nock does not work for fetch requests).
The solution is to use a MockAgent that is built into the undici package.
Let's say your code looks like this:

// constants
const U = 'http://127.0.0.1:5984'
const DB = 'users'

const main = async () => {

  // perform request
  const r = await fetch(U + '/' + DB)

  // parse response as JSON
  const j = await r.json()
  console.log(j)
}

main()

This code makes a real HTTP request to a CouchDB server running on localhost.
To mock this request, we need to add undici into our project:

npm install --save-dev undici

and add some code to intercept the request:

// constants
const U = 'http://127.0.0.1:5984'
const DB = 'users'

// set up mocking of HTTP requests
const { MockAgent, setGlobalDispatcher } = require('undici')
const mockAgent = new MockAgent()
const mockPool = mockAgent.get(U)
setGlobalDispatcher(mockAgent)

const main = async () => {
  // intercept GET /users requests
  mockPool.intercept({ path: '/' + DB }).reply(200, { ok: true })

  // perform request
  const r = await fetch(U + '/' + DB,)

  // parse response as JSON
  const j = await r.json()
  console.log(j)
}

main()

The above code now has its HTTP "fetch" request intercepted with mocked response.