How to mock location inside storybook stories?

If your component is expecting to be wrapped into Router you can mimic this by wrapping it into MemoryRouter inside your stories:

import React from 'react'
import { MemoryRouter, Route } from 'react-router'
export default {
  title: 'UserCard',
  decorators: []
}
const UserCard = ({
  match: { params: { id } },
  firstName,
  lastName
}) => (
  <div>{`${firstName} ${lastName} ${id}`}</div>
)
export const Default = () => {
  const props = {
    firstName: 'Al',
    lastName: 'Pacino'
  }
  return (
    <MemoryRouter initialEntries={['/user/108']}>
      <Route
        component={(routerProps) => <UserCard {...routerProps} {...props} />}
        path='/user/:id'
      />
    </MemoryRouter>
  )
}