I don't think you can really generalise that way. Someone who writes good code will write good code, and someone that does badly will write badly regardless of passion. Someone who is not paid is going to be more likely to complete something they want to make than something they don't, but if they are paid they don't have that problem.

People writing banking software, or satelite control code are very rarely passionate about it, but they have very strong methodologies and testing strategies that ensure that their code will be of a very high quality.

It won't generally be the best application, but it will fulfil the specs and be reliable.