When people call it one of the worst mechs ever, it becomes fair to add context. The post saying it would be a fair match to have the Jager go up against the Stinger may have been using hyperbole, but it fit in context by calling it one of the worst mechs in 3025. Mech v mech is not the only thing in Battletech afterall.
Yes, the post was "hyperbole", in that I wouldn't really want to take a lone Stinger up against a JM, but I WOULD try it with a basic LCT-1V Locust (DID it in a multiplayer game, actually, and it wasn't exactly pretty for the JM).
The JM is a highly specialized AA design at best, in a role which I feel could be done much more effectively and cheaply by a vehicle instead of a 'Mech. Both the cheaper and lighter RFL and BJ already cover the AA 'Mech niche almost as adequately, in my opinion, and are more than a match for the JM in almost any other role.
The JM has enough firepower at range to make it beneficial to take out as quickly as possible, and not even close to sufficient armor to weather the special attention that it will generate. In short, if it's ever in anyone's weapons range, it's going to die quickly and horribly. It's most appropriate place on the battlefield is....just off the edge of the actual battlefield, where it can do its job in at least reasonable safety.
The hilariously bad matchup as originally posted was a case of taking a fish out of water and pitting it against something that naturally eats fish.