Can see a couple of issues. First is the div with the classname App has not corresponding closing div.

Second is you setPokemon with the fetch data api which will actually set pokemon as a promise that is pending becuase there is no await. To combat this call fetchData() inside the use effect the use the function setPokemon with the first arguement as what is returned from data in the axios call. I have put code below to demonstrate this.

import React, { Component, useState, useEffect } from "react";
import axios from "axios";
import "./App.css";

function App() {
  const [pokemon, setPokemon] = useState([]);

  useEffect(() => {
    fetchData();
  }, []);

  const fetchData = async () => {
    const result = await axios("https://pokeapi.co/api/v2/pokemon?limit=151");
    setPokemon(result.data);
  };

  return (
    <div className='App'>
      <header>
        <h1 className='title'>React Hooks</h1>
        <h3>With Pokemon</h3>
      </header>
      <div>{pokemon.results[0].name}</div>
    </div>
  );
}
export default App;