As stated above, use a class with Cold Blooded if you are having problems with players using LMGs and target finders. You can also easily snipe them from a distance or flank them with a more mobile class because these players typically don't move much. Tactical grenades are also useful.
Flash Grenade and EMP Grenade screw over the Target Finder pretty well. Coupled with Cold Blooded, and it's pretty easy to get right past them.
I do agree that the Target Finder tends to be used by those who either can't aim well for whatever reason (just plain bad or something) OR easily get target fixation; just stare ADS and usually lose sight of their surroundings easily in almost every sense, thus they rely heavily on the diamond highlighting flashing to get their attention. It's why using Cold Blooded can, sometimes hilariously, let people walk right past the user on their left or right side and they'd be none the wiser because they're so busy staring straight ahead at the center dot or waiting for the flash of highlighting diamond(s) to alert them.
I still don't think it's OP at all though.