Currently, every time I want to update the decorations in the view I send a transaction though the editor state. Is there a way to trigger updates to the decorations without a transaction?
Even if there is a way, seems you probably don’t want to do this other way… 
(Most probably somebody will ask you “What do you need it for?”; so you could give some background to help others address your problem!)
No, this is how ProseMirror works—state updates go through transactions. You’ll have to address whatever it is that is making it a problem to dispatch transactions, rather than dispatching less of them.