The biggest issue with Skull Bash's usefulness is its startup and ending lag. Against a good opponent, if you don't hit with Skull Bash (which you won't 95% of the time, because a smart person will powershield it, spotdodge it, jump over it, or use a projectile to stop it), there's more than enough time for them to punish you. If you're moderately far away, they can just dash-grab you, or at the very least use the time to space themselves and put you in a bad position.
That, and it's super telegraphed. Even if you try to trick someone by releasing it sooner/later than they expected, there's two scenarios: 1) you're far-ish away, in which case they'll have enough time between the release and your getting to them to react and dodge/shield, or 2) you're close, in which case they'll attack/grab you out of the startup lag or charging animation.
I love Skull Bash too, but the fact is that offensively, it's very minimally useful. It can surprise people sometimes, but after you get them with it once (and chances are it won't be anywhere near a fully charged one), you won't get them with it again. The only time I've ever caught people with a charged one is when I pass the ledge on recovery, usually by accident.