A generally likable horror flick save for the downer of an ending. Without giving anything away, the only rational explanation I can think of for the ending is that the producers were trying to set up the audience for a sequel?
I know Smile has been compared to a previous movie, "It Follows," but I think that, thematically, it more follows in the footsteps of the 2004 film "Session 9," since both films turn mental illness into an actual malevolent entity that preys on vulnerable-minded people.
Back to Smile, the only take away from the ending is that if you've experienced trauma in your past, you're only destined to become either a psychopath or a manic depressant as a result. Forget therapy or finding a way to use your past to make a better future, the lovely message here is that murder or suicide are the only two fates that await you if you've dealt with trauma in your past.
That said, I'm guessing the sequel will be coming out next Halloween or the Halloween after.