Maybe its bad idea, I dont know
Sort by Post count?
Post Count, Trust score, Merit are basically variable numbers.
On the table whatever post count you give to a users, once they make one more post (or any of their post gets deleted) then the information on the table turns out to be wrong. Same with trust score and Merit. Once the number changes then the information is not longer valid. A visitor can easily see these by visiting the profiles of the users, they see the correct information every time in this case.
Sorting by username (alphabetic order) is the best practice I think. This way you do not give any user a biased order plus if anyone wants to find a specific user then it's easier for them once the list turns out to be a long one.
There are mainly three features: Signature, Avatar, Personal text. So, I thought to have separate columns for each of them is more appropriate to show their status instead of one column for all three entities.
Here what I executed as example. Feel free to use it if you want to.
Anyway, it's your post. You are free to find the best design layout for you 🙂