I think it should be based on the crowd solely as that's the only thing that seems fair. However I think there should be different "states' in which it becomes a factor. Whenever the crowd cheers it should pop up a "condition" which has to be adhered to for you to use the smash move. It fades away with the crowd however so depending on the condition and the loudness of the crowd it can last for longer or for shorter. Some of the conditions could be as such:
"Underdog"
Smash item appears on stage HOWEVER it can only be acquired by a character if they were the last person to recieve a hit within a few moments of grabbing it - which means if two people are brawling over it, only the on that's on the run can use it.
Or
If one player has significantly lower health + lives than the other but has performed well enough to warrant praise, the "underdog" condition could be activated... HOWEVER this condition carries with it a price - should they fail the conditions requirements (like perform a fully charged smash or not be killed for a set period of time) then will in turn their OPPONENT will recieve the super move and their second chance will be spent.
I also think that this condition should have to be "accepted" by the underdog for it to occur at all.
"Overkill" or "Mercy Rule" condition (or a nicer term for it)
One player has outperformed the other so significantly both in amount of life remaining, technical score, that the crowd plays the "mercy rule". The mercy rule either causes the move to occur and end the match, or the other player gets a free super smash move to counter the landslide. This could also only be accepted in the first case by the stronger player, or by the weaker player - they'd have to "confirm" the rule in case they didn't want it.
"Bully"
One individual has used a number of "tricks" to gain a high score (an abundance of "item kills", spike moves, and such) and they get a free super smash. The crowd may or may not boo when it comes up. However accepting the "bully" condition comes at a price - you lose a life, or it may backfire resulting in the crowd cheering. Generally it should fail and just be there to annoy people
.
"Sacrifice"
You recieve the opportunity to sacrifice a life to pull off a smash move - this will not end the match should you eliminate with the move but it can miss resulting in your failure - this should be activated and then you should have to fulfill a requirement for it to occur or not - should you be unable to do the "sacrifice" condition you'll lose a portion of life and afterwards the crowd will not cheer for you and thusly you will recieve no other smash moves till the end of the match. The requirement could be landing a fully charged smash move or possibly living a long while, or simply putting 50% damage down on an enemy in a specific time limit. It's sort of a match ender if you're good. It could be initiated by a number of conditions, like your life exceeding 300% or being on your last life in a team battle.
"Sudden Death"
In a 1 v 1 if the life of two individuals health exceeds 300% it is reset to zero and the characters are made near impossible to knock off. However it becomes a stamina battle henceforth - the first player to recieve 111% life will receive the bad end of a smash and it is unavoidable. Items are turned off during this condition and it is automatic - players do not need to confirm it as it is difficult enough to achieve in a real match.
"Chance"
In any match if a certain condition is met, a special condition like all players dieing within a certain timeslot, if all the players confirm it then a die is cast or a special number displayed and a random condition is instilled on a player. Special "Chances" could be like "sacrifice chance" or "penalty chance" which instill a specified condition.
It's a start, I think a lot of this is based too little on skill... will have to revamp later.
Speaking of "later".