Material UI v5+
You should be able to globally customize MUI component styles, such as:
import { createTheme } from '@mui/material'
const theme = createTheme({
  components: {
    MuiLink: {
      styleOverrides: {
        root: {
          textDecoration: 'none',
        },
      },
    },
  },
})
const App = ({ currentAccount, neighborhoodsWithPropertyCount }) => (
  <ThemeProvider theme={theme}>
    <Router>
      <Routes>
        <Route path="/" element={<Home />} />
      </Routes>
    </Router>
  </ThemeProvider>
)
export default App
However, more often than not, you should actually be using the Link component from react-router-dom, in which case links would have no text decoration by default.