80-20 rule is more like 80 percent of your business value comes from 20 percent of your features. For a particular feature, 20 percent of the effort may get you more than 20 percent of its business value, but it will likely get you technical debt, and skimping on UX can actually hurt the business value of other features.
A good practice is to have some DoD in place for both coding standards and design standards, then try to meet them with the minimal amount of effort for each feature.