If the sidebar block in a report would also display the number of previous reports about the member, then it would be easier to spot problematic users.
If this would also be displayed on profile and member card, then this would make things easier for moderators.
Don't get me started on the xenforo report system, while it's great to have, it's terrible work with on big boards. Almost any board where we support user reporting content with 500+ online people, we force reports to threads. Because we can search through them a lot easier. They are visually there, and there's no week by week or month by month cycling through things from 2 years ago. You just sort on username for that forum and have all their reported content listed in 1 go. Easier to merge content as well with dupe reports, or just to archive the issues of a user who ended up getting banned. The current report system is a lot of extra clicks and reading what's actually on the screen.
Having extra features for staff like 'this user rating is below average' so we know their comments are already bending the rules, have been reported before and should get looked into, or justify a warning 'this time'..' really helps, as well as what you suggest, where we can optionally display a number to show reports (valid or not, solved or not) against the user('s content). Together with seeing a profile in a different color or with a notice above it (or their threads, posts, profile posts, conversations, etc) that have been reported in the past or recently. Would really help.
Right now it's dumpster diving experience, which is so time consuming that most mods don't even bother. Or active boards just choose an alternative method to dealing with reports (as a ticket, as a thread, as a group email, ..)