marth's tipper is insanely easy to hit. just because you cant doesnt mean zelda's is easier. marth's should be easier just because his covers more area than zelda's though basically the size difference is negligible (seeing as the majority of the slash doesnt matter). if you want me to pull out hitbox pictures to prove this to you i will.
I'm not talking about sweet spot area actually, and yes I CAN sweet spot with Marth (he is one of the characters I use). Marth's F-smash however has considerably more lag, at least before it hits the important area of the slash (well, if you're hitting people in front of you anyway), which leaves more time for them to do something that changes it from a tipper to a regular f-smash, which is why it's harder, you need more time to properly execute it. Lightning kick is far closer to instantaneous, ESPECIALLY reverse lightning kick. Really those couple of frames make all the difference.
That said, I'll gladly take you up on the hitbox data anyway, I've been looking for it (in general, not just lightning kick and Marth's F-smash).
btw, leeway on spacing is very high especially with the top tiers. other than marth none of the top 5 chars can be shield grabbed unless they **** up. with marth your whole game is spacing so it doesnt matter. zelda is bottom tier because she sucks. she has things she can do, dont get me wrong, but shiek beats her in every catagory, just like she beats 60% of the cast in every catagory. you know how some people say mario is middle tier cause he can do everything all right? well sheik is the same way, except she does everything really well. she has ganon's platform game, one of he best projectiles in the game (has the highest priority, even cancels out one of falco's lasers, and 1 needle blocks full charge shot), no brain fairly inescapable combos, and tons of finishers. every one of her moves can be well put into her game, least used of which are fsmash and side b, but both having their uses (unlike other top tiers, like marth's usmash, fox/falco's fair, peach's b/sideb, ect) she can kill off the top like fox, do marth's gay off stage gimping, almost no lag, the only thing she is missing is a spike. zelda has what? a dsmash and 2 aerials.
I know about leeway on spacing, but everyone messes up occasionally, and higher tiers aren't all you might have issue with (think Ice Climbers, though any grab-based combos aren't too effective on them unless nana is ko'ed).
But you must realize that I don't essentially disagree with you, my caveat is with your absolutist wording, you say Sheik does "everything" better then Zelda as opposed to "very nearly everything". That, and my belief that those cases not covered under "very nearly everything" merits an occasional change.
What does Zelda have? Well, as you pointed out the dsmash and 2 aerials which outprioritize God (ok, maybe not, but they have VERY good priority), but she also has a pretty nice f-smash (which even if you don't sweet-spot it has considerable knockback, and has nice damage in any case), another good aerial (up-air, though you have to time it well, the massive hitbox does certainly help make up for it, not many d-airs can get through it if timed properly, Marth's might, but I don't think so), a projectile reflector which doubles as an egdeuard, an ideal up-smash for comboing at low percents, good recovery, and the act that her up-b can be used to move very quickly from place to place on the stage, and is near lagless if you land on the ground.
Sure, it's nowhere near as impressive as Sheik's list of ailities, but they're useful.
Oh, and Up-smash for marth has it's uses, if only the threat of it being an incentive to not be directly above Marth, though it's rare that up-tilt isn't better. So does Peach's side b, however that's many recovery. Not being a fox player I haven't really explored enough to find uses for his fair.