"Here is Sub-Zero."

same

imma guess terrible writing

I always took it as " Plane " opposed to a sub.
I don't know though.

In this context it meant that Sub-Zero was now just zero, as in nothing, since he's dead.

Yes, using the modifier "plain" in place of a previous modifier lowers the significance of the thing being modified.

Here are three items from the definition that fit:
Obvious to the perception or mind; evident: synonym: apparent.
Straightforward; frank or candid.
Common in rank or station; average; ordinary.

He's no longer Sub-zero, he's now just plain zero.
Obvious zero.
Straightforward zero
Ordinary zero

In other words, he's no longer Sub-zero the successful stalker, he's just a zero. A plain zero.