My girlfriend and I travel in premium cabins for all medium and long haul - we usually turn up wearing athleisure wear and occasionally other passengers think we are in the wrong check in/bag drop queue. Probably people making assumptions based on our attire and young ages. I’d rather be comfortable than meet the expectations of strangers.

How you dress will probably make no difference to your upgrade chances, as it’s mostly automated now. I imagine it would be far more effective to be good looking than well dressed, if any human decision making comes in to the upgrade process.