Mega-CP is/was real, and there was an officer who was the Mega-CP, but they're presented as both a superhero like Superman and one of those humble 'good example of (occupation)' like postman pat or fireman sam. Mega-CP was used as a propaganda tool (directed at both the citizens and fellow Civil Protection) by the UU in order to act as the posterchild for Civil Protection, portraying them as a role model for Civil Protection who helps out the community and enforces the 'good' of the UU. For public events and speeches, Mega-CP would often attend in the same way Ronald McDonald does, wherein there are many different actors (all officers) who play the role of Mega-CP but that anonymity makes it incredibly easy to just give an officer a badge or sash and have them play that role without scrutiny.
Mega-CP had action figures (both of Mega-CP and the 'villains', including enemies of Breen like Dr. Kleiner and Eli Vance who in the series were portrayed as evil mad scientists) a few comic book lines for the Loyalist cities and the private distribution network for Civil Protection, and was used for a lot of branding on anything Civil Protection related, similar to how Uncle Sam is for the US.
In the post-occupation era, Mega-CP went out of production and the majority of products either with Mega-CP branding or Mega-CP affiliation/propaganda became collectors items both amongst Civil Protection and Rebels alike, one side coveting them as their 'Freeman' and the other finding it funny to mock.