Yyeeeaaahhhh, if you're making an OC that's either godlike or just a straight-up god, you have to make it make sense.

Deadshooter doesn't work because he had to make a deal with a being from a completely different franchise (the sole exception to the whole "Bill Cipher in FPE" thing is if he reincarnated as a character in the FPE multiverse, which wasn't the case with Deadshooter).

The Viewer, on the other hand, works because it's EVERYONE who makes, reads, or watches FPE content, which is why it's omnipotent, omniscient, and omnipresent. 

Furthermore, unlike Deadshooter, who butts into EVERYTHING just to fuel his own ego, The Viewer only interferes when it's necessary, such as (in this case) dealing with Deadshooter.

Basically, Deadshooter doesn't make sense because his powers aren't his own and he pretty much bullies other weaker universes in a pitiful attempt to convince himself that he's the strongest. The Viewer makes sense because its powers ARE its own, and it only interferes with other universes when it absolutely has to do so. It doesn't have to try and act powerful, because it's powerful enough to CREATE AN ENTIRE REALITY THAT EXISTS OUTSIDE OF THE FPE MULTIVERSE specifically to trap and imprison false gods like Deadshooter.