Commercial success definitely increased after he left (though he was the driving force behind getting them to the big stage in the first place). But I would say that the originality and creativity of the Piper at the Gates of Dawn/Saucerful of Secrets era was well beyond anything the band did later. My favorite PF tunes are all from those 2 albums.
Of course later on, Barrett really did lose it, and if you listen to his solo stuff, there are flashes of brilliance, but it is too far gone to be consistently good.