Good software doesn't need to look good, though. Looking good is just a bonus. I don't care in the slightest how it looks because it's still better than the alternatives.
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.
Given the laundry list of issues they need to resolve, and as apparent from this thread the choice by UI experts to complain on HN instead of leaving feedback, I'm absolutely fine with them prioritizing other issues which are easier to address and have far more of an impact on their users.
A well designed UI also makes it easier to understand the UI, what belongs together, where one area ends and another area starts.
Tbird's ongoing lack of polish is a huge chunk of why I don't use it.