Is going on a different type of build get you higher chance of getting an S-grade?

dont know... but here is a thought to ponder....

stop worrying about the grade and try to have fun and enjoy the game

that is what is here for

If your previous build sucked, then yes. But your build is not used in the calculation of the grade, if that's what you're asking.

going an off role does though

I'm pretty sure the grade is based on the average performance of other people playing your champ on that map.

For example, Kog has something like a ~33% win rate on Twisted Treeline and I've found ways to make him do decently there. As a result, I've been getting S ranks in three out of every four games.