Should I keep this surge suppressor in my main load center?
They are intended to be mounted outside the box...
You are correct that they are intended to be mounted to a KO (usually on the breaker box), instead of being left inside, all flop-a-dop, as yours was.  You can use this unit still, although you will probably want to leave some space around it, as its further lifespan is unknown.  (All MOV-based suppressors have a finite life; a MOV that's seen too much will break down under normal mains voltage, which on good suppressors will cause a thermal fuse to blow and take it out of the picture.)
...and not double-tapped either
While you're in there, you'll want to clean up the &quot;double tap&quot; on your main breaker's lugs and move the surge suppressor wiring to its own two-pole breaker of an appropriate ampere rating, or use wirenuts and appropriate-gauge pigtails to add it to an existing 240V circuit of a suitable ampere rating.  Note that Siemens breakers (unlike Square-D QO and HOM and Eaton CH) are not rated for two wires under a branch breaker lug, by the way.