As a guy who really wanted Isaac in both Smash 4 and Ult, I'm starting think Isaac did far worse than what we all thought. Obviously he didn't touch K. Rool levels, but looking back at it, the other characters that got due to demand in all had something going for them. Obviously Ridley and the Belmonts had the ballot, but Ridley also had the power of memes on his side (and don't say memes aren't relevant, the "TOO BIG" memes is probably one of the biggest factors on why Ridley was relevant for so long), Chrom also had a big following during Smash 4 as well as the having power of memes, DS was the main failsafe in case Ridley didn't work, and Inkling were not only popular and relevant, but they were amongst the highest requested characters since Splatoon was revealed... in 2015, when Smash 4 DLC was at it's peak ( in fact, Sakurai admitted they were a given).
If there were another ballot, he would place higher, but I have doubts that he would place in the top 5, considering that he probably didn't in Ultimate. I can see Dee and Shadow making it in top 5, but not Isaac.
Waluigi would probably win though, considering his publicity is INSANE, like, way higher than Ridley levels.