I have this issue too, but in my case showing the avatar of the topic starter wouldn’t solve it.
Users receive PMs with fixed titles as a product of one workflow and I’m included in circulation(1). A number of other PMs are generated from links, as a result they have a standard title that many don’t customize(2).
The inbox view on mobile is just the title and age. Sometimes that means screens of the same-titled topic with zero visual differentiation.
It’s almost impossible to tell like-titled topics apart from this view. Seeing the topic starter would only solve (2) - the avatar of the other participants would cover both.