I love the service, but it could do a better job of following Google's material design. When I compare the UI of Google Analytics and PIWIK, IMO Google does a better job of applying their design language. I'd like to request that the side nav, graphs, icons, and search bar be updated to look similar to what I've found here. Thanks for reading my request.
May contain own opinion: Oh god, please no!
As design is always a bit subjective it should be possible to develop an alternative (more material design) theme (see https://developer.piwik.org/guides/theming) and publish it on the marketplace so everyone can choose.
Can you maybe suggest some concrete improvements ideally with screenshots of what it should look like? This will make it easier to improve it. Otherwise we might need to close it.