Even by the whole "expendability" argument, Mewtwo would be the most "expendable" by virtue of being a character who was cut in Brawl due to low priority and, despite his fan demand, only managed to return as a DLC character.
Mewtwo was formerly expendable but Sakurai most likely views him different BECAUSE of his prior cut.
Jigglypuff doesn't have that same luxury. She's squeaked in a few times now. She barely made it into Brawl herself.
So by using some mental logic (that could be wrong mind you), Sakurai is probably mindful of the fact now that Mewtwo caused an uproar cutting him, realizes his popularity and has placed him in higher priority.
Now you could wager hype has died down a bit for Lucario, but he's still got a far more sizable franchise fanbase than Jigglypuff does.
Greninja is still pretty fresh. Charizard is still stupidly popular after a bunch of generations passed. Pikachu is the actual mascot.
Jigglypuff is someone who is no longer as popular in the Pokemon fanbase as she was back when she was first conceived for Smash 64 (where she got in because of her similar shape to Kirby after all), and has had low priority for Sakurai ever since.
It's a likelyhood that's real, homeskillet. She's low priority and could get cut in the event of a necessary Pokemon cut. She'd cause the least amount of damage to representing Pokemon's legacy.
The thing that potentially will/would save her is the fact she has that original Smash 64 legacy going for her.