I see a lot of blow-outs in the playoffs...but if it were as you suggest, that would also be the logical, as well as the most emotionally understandable result: You want to regard yourself as a rival of great teams, not mediocre teams.
If you beat a mediocre rival, what do the fans get to call out? "We're MORE mediocre! We're MORE mediocre!" It doesn't really have a ring to it.
I belive rivals position in league isn't inportant.
Fan will simply go WE BEET RIVAL!!!! and that is it, no mather if rival is 1st or last in division.
As someone point out rival are team your team (ora at least fans) hate.
By my oppinion winning a game against rival should get more attendance (or at least merchandice) boost than beeting 1st in you division. Shure if rival is 1st even better. And also beeing ranked in front of your rival in regular seson (at the taime of survey) should give some boost to fans too.
And one question how is rival determined for tems that promote/demote? Random betwen promoted/demoted teams, or random between all teams that do not have 2 rivals, or something else?