The epilogue, like the epigraph, serves to really hammer home the "hidden moral" of the story, which is basically drugs are the most terrible things ever.
By ending the diary with an editor's note about how poor little Alice finally succumbed to the siren song of hallucinogens, her story is now one to learn from: If you do drugs, they will eventually kill you. So don't do drugs.
Unfortunately, ending the book this way feels unnecessarily harsh and abrupt. It's like someone yanks the book out of your hands and slams it shut in your face, saying "There. It's over now. Go do something else." (But not drugs. Don't do those.)
However, the authors had kind of written themselves into a corner. They couldn't very well have had Alice write it, since you can't really write about your death-by-overdose while it's happening. And they also couldn't have the story have a happy ending—this book is supposed to discourage drug use and sexual promiscuity, not teach you that it's okay to have a type of hippy Rumspringa. So the authors were forced to take this tactic, but you can see why it is far from ideal. At best, it's just a little heavy-handed.