There might be a bit of bias. But this is for obvious reasons. The great majority of people in this country, and logically, football fans are Christian. Also, Christianity is based a lot on being a good person, e.g. Sermon on the mount, Golden rule, etc. Therefore, if one claims to be of firm Christian faith, we generally view these people as being good guys, since we are also Christian (we project).
Lets complicate this a bit. Does race play a role? Some of the worst or "bad guy" characters in the NFL (Owens, Ray Lewis, Cam Newton, Marshawn Lynch) also claim strong Christian faith, but are never perceived as "good guys".
Personally, I think it is a combination of faith, race, and general congenial demeanor. There is a romanticized collective vision of the "good guy" that has never been historically associated with anything bad or evil. It's an archetype. White, Christian, Protestant, honest, loyal, hard-working, family oriented, articulate, neighborly, duty, etc. Peterman and McDermott both epitomize this archetype.
It reminds me of an old Onion article:
Pro Athlete Lauded For Being Decent Human Being
See the connection to race in this satirical article?