I think it means "does it look like an app that you would trust with your email workflow".
If the design looks broken or amateurish, maybe that would lead you to not trust the application because you feel it is poorly made and will lose your data one day.
Or maybe the new design is so focused on stickers & gifs* that you don't trust the application to reliably handle your professional business communications with clients anymore.
* (I'm looking at you, Skype, with this comment...)