The collective record of Rams' opponents this season is 20-27, which isn't terrible or fantastic. If you take those teams' records outside of their losses to the Rams, they're .500.
However, the Packers' collective opponents have gone 17-21-1 this season, and the Packers are only 3-2-1 (as opposed to undefeated). So I don't know if this angle is especially telling. Yes, the Rams have played some terrible opponents (Oakland, Arizona, San Francisco), but they've also knocked off the Chiefs and Chargers, which are way more impressive wins than anything Green Bay has pulled off. I guess you could argue that the Packers had the Vikings beat and it was taken away, but they also let the Vikings' offense walk all over them after they got screwed over, so that rings kind of hollow.
I give the Packers a chance to be sure. If Rodgers is his healthiest since the start of week 1 and the offense is clicking, they'll cover and could absolutely win outright. However, if it's more of the same from before the bye, they're going to get boat raced. All things considered, it makes sense that they're heavy dogs in this one.