Honestly, I've gotten more KOs with Fsmash in neutral than seems reasonable, just because it has more range than Inkling's normals and can catch people out of a dash dance (or to punish things like a heavy or swordie's tilts). Landing a grab or a roller seem like they're almost necessary for setting up kills, be that up throw -> up air (which is a true combo that kills on certain characters at certain %s, but I haven't played around with it enough to know much more than that), or roller -> Fsmash/Up Smash. That being said... Inkling does a great job of getting incidental kills by just leaning into Ftilt, Fair, and especially Bair a bit more as %s get higher. If you're ahead and find that you can keep getting free % by playing safe against someone at 100%, it's not wrong to just land more ink and combos to put them firmly in "dies to everything" ranges, if you must. If they're anticipating rollers and grabs and Fsmashes, going for Nair combos and neutral B to get a stray kill off of Ftilt or something doesn't feel too bad. Oh, and also remember that splat bomb can definitely kill at higher %s, as can Fair/Bair near ledge, of course.
Ultimate's weaker defensive options feel like there's less need to necessarily set up one's kills, though-- especially since a lot more normals feel to double as high % kill options. Not sure I've seen any stocks lasting past 140%, and passing 120% feels somewhat rare anyways, whereas in Smash 4 I might see 180% regularly if I had no specific setup. That being said, I think "get stage control" is a great wa to set things up, as it means incidental KOs with moves that won't normally kill are just that much easier.
Fsmash, often from pivots or roller, definitely feel like the source of most non-ledgetrap KOs I see, though.
