Filter Comments/Jokes/Memes from actual reviews and introduce "helpful"-button
Make a filter to switch user input between Comments/Jokes/Memes and more serious reviews.
I suggest to give Comments/Jokes/Memes a reduced number of characters so that they feel like a tweet on Twitter. Keep the "Like"-Button here.
Actual reviews with great and insightful content should require an appropriate minimum of characters before being posted. Introduce a "helpful"-Button here.
Abusing the review section for Jokes should be reportable.
I think this is a solution that would benefit everyone and the social aspect of letterboxd would not suffer.