The Mascot Horror genre relies a lot on having an eyecatching design for its primary marketing campaign. This is because the quality of a mascot monster design comes down to how close it is to being scary while simultaneously looking like something that was genuinely designed as a pure toy. You need a degree of originality and subconscious repulsion that is not found in more conventional Horror works who only worry about the first part.A good mascot that toes this line is therefore a good sign for the game and players know this.

For example, this is why Huggy Wuggy is one of the greatest mascot of the genre, he looks like a normal toy until his mouth opens.

All of this encourage the devs to put the design they are the most confident in front and center. For Secret of the Mimic, the Mimic himself is not cute/doesn't look like a toy, Big Top and Moon looks too weird, Nurse Dolly and the other costumes are too blands (though I think they could've done some clever marketing with them), and Red Tiger doesn't look like a robot at all, which is part of FNAF's identity. Jackie was the most suited for the role to reel people in, not to mentions clown being scaries is still a thing a little bit.