AOC is completely correct. If you only show up every four years to run for an office you haven't done the work to win, your purpose is to act as a spoiler candidate. Jill Stein is an unserious person who doesn't care about winning.
There's nothing wrong with third-parties, but if you're just running for offices you literally won't win, you're a joke. Greens can run and win in down-ballot elections, but winning the presidency is a farcical idea until they actually put in the work to win in more local races.