if you want a technical answer you should read 'The Didgeridoo Phenomonon' (SP?) but if what your looking for is a validation that a smoother interior will produce a cleaner sound wearas a rougher surface would be muffled, then you are mostly correct. I wouldn't use the word muffled but I'd say warmer or dirtier. And you might notice I dropped out the 'better' that was along with cleaner in order to back up your comment, because 'better' is purely subjective to the critics preferances. In a straight cylinder the sound waves would maintain a similar pattern as it travels through the column with those wave lines intersecting at the same place in each pattern causing cancellations of certain aspects of the tone. Bumpy walls would create disruptions to those patterns causing irregularities to the wave patterns in terms of locations of intersecting lines and therefore the cancellations of certain aspects are only partial rather than total, creating a more colorfull palate. Pretty much only pvc players are playing on straight cylinders, for the rest of us, the fact that there are irregularities to the air column itself boggles the calculations and this simple theory talk is good for simple theory, but truely trying to understand it beyond that is for a select minority of freakishly gifted intellectuals. Finding your own preferance for dirty vs. clean will come as you develop your own style of play, you'll use what best supports your sound.
Here's something related. Drum machines are capable of delivering a very precise beat but are actually programmed to develop a slightly wavering beat because that exact precision just sounds too artificial, so they give them a slight bit of imperfection to add a more human element.