when you click on the avatar or nickname of the post’s author, you see his user card, which gives some information about the user (incl. the join date). For the number of posts, you have to click once more on the avatar or nickname (the one you see in the user card) and it will open the user’s profile in which you will find more extensive information about the user (incl. the number of posts).
There is no option to display these informations directly in the thread. But Discourse is open source so, try to see if this kind of feature has been requested by searching in meta.discourse.org and if it hasn’t, create a new topic there.
That’s too bad indeed. A couple of posts above there is “Migration: Gold Supporter”. Was hoping there was a way to customize what info is there. If I have to click once, more, or many times, it’ll end up being ignored instead as that is simply easier. I’m happy to help, but not at the cost of having to do a lot more job myself (for every top poster there is).