Seems like you could use this: github.com/PirouzSR/jellyfin-letterboxd-ratings

You would need to use either the “Write to Critic Rating” setting (if you want to get rid of the Rotten Tomatoes rating and use Letterboxd instead) or the “Overwrite Community Rating” setting (if you’d prefer to get rid of the IMDB rating and use Letterboxd instead of that, which I think is what I’d do. IMDB ratings are so unreliable).

That plugin will add the Letterboxd rating to the end of the movie descriptions as well, but that can’t be used for sorting, whereas critic/community rating can be used to sort in all the official Jellyfin clients that I’ve tried out.