If software doesn't appear "trustworthy" people won't use it, particularly if it's holding sensitive personal data such as email. The worst UX is people never using the software.

Design plays a big part in how people perceive trustworthiness. It implies effort and understanding. The reason people spot fake products on AliExpress is not because they're intimately familiar with the legit versions, but because typically there are numerous design aspects that are just slightly weird.