It's both a natural thing and a psychological thing in my opinion.
The way I look at it is that with a higher quality instrument you're going to produce something that is of higher quality. If something is just easier to play, it suits you, it feels good to hold, the strings are just the right gauge, the action is at a height you're comfortable with then you're bound to be better, without a doubt.
As for the psychological thing; I've felt much more confident on basses that are aesthetically more pleasing. It's that feeling of knowing you look badass with what you're holding and that confidence is something that you can take and run with. I used to have a Peavey Millennium BXP, don't get me wrong it was a nice bass, looked good. A few years back I sold it and got an Ibanez SR 1605 and the difference I felt as a performer was unbelievable. I know this may come across as potentially sounding arrogant but when I hold that bass on stage I know that it's appreciated and what that does for me as a performer really makes the difference.