A clown would be fine but I would go with one of the more aggressive species like a Maroon, Tomato, Cinnamon, or even Clarkii. Also keep in mind that a Picasso Trigger will not be able to live out it's entire life in a 55g and you will need to upgrade eventually.
You will also not be able to keep any sort of clean up crew (snails/crabs/shrimp) with a Picasso Trigger, it will eat them all. Some (including me) have had luck with Longspine and Pencil urchins to deal with algae.
Some will also tell you that at around 3" a Picasso Trigger will decimate everything in your tank, and it is entirely possible, but many others have found their Picasso on the more docile end of the trigger fish spectrum. If you are interested in a CUC and even corals in the future you may want to consider a trigger with an upturned mouth (from the Xanthichthys genus) such as a Sargassum or Bluethroat Trigger but any of these will eventually get too large for a 55g tank (years).