I have to agree with Scav on this one. Honestly, all of Mario's high-knockback moves (d-smash, u-smash, b-throw, sex kick) are all either situational (like u-smash, which works on low-ceilings/floaties), have relatively low knockback (sex kick/d-smash), or can be DI'd so that the move really isn't worth using if you want to net a KO (b-throw).
F-smash is pretty much the only thing Mario has, and even though it's reasonably effective as a KO move (despite the fact that it can be easily DI'd and has an annoying sweetspot), the main problem is that setting up for a f-smash is nigh impossible.
Proper DI (so long as you DI away from shffl'd u-air and down) means that landing a f-smash, up-angled or not, is pretty much out of the question. Combo'ing into it is incredibly difficult because of this, and even when you can combo into it (i.e. d-throw -> f-smash on Sheik, Ganon, YL), sweetspotting and DI makes it completely unreliable.
So, just like Scav says, people basically have to want to get hit by f-smash, from a certain point of view. It's basically just a version of Roy's f-smash--the only reliable way to hit with it is to simply mindgame it into whatever you're doing. When it lands, you're ecstatic. Otherwise, the move itself just plain sucks. Which basically describes Mario's f-smash to a tee.