Saying sheik is better because you can make mistakes and still win is WAY off base
Thing is, it really isn't at all off base to say this because that's how all fighting games work: two players attempt to make attacks that will create openings for them to deal damage, and they attempt to remain as unpunishable as possible while doing so.
When it comes to doing this, it's expected that a certain number of attempted attacks will succeed while others will fail during the normal ebb and flow of the match as each player attempts to read the other's intentions and counteract them.
The problem with the Shiek vs. ROB matchup (and largely with Shiek in general) is that the player using Shiek can afford to make many, MANY more attack attempts with very little risk involved, putting less effort into reads as well because Shiek's moveset allows her to attempt to attack and withdraw quickly if the approach turns sour.
Meanwhile, the ROB player is at the exact opposite side of this spectrum: every attack ROB attempts to make has a real danger of not only being punished but being punished for a sizable amount of damage. Shiek is also much more difficult to read than ROB, largely thanks to her superior mobility.
Of course, a ROB player who is just that much more skilled will beat a Shiek player who lacks the same skill. No one is arguing that.
The trouble is that the amount of skill advantage needed by the ROB player to defeat the Shiek player isn't typically found at the highest levels of play, and furthermore, if a player is THAT skilled with ROB, s/he'd be better off playing Shiek anyway because Shiek's superior frame data gives them that much more of an advantage.
So yeah, characters with safe options tend to rise in tiers because they can afford to make mistakes and the skill of the player using them can push the character much further.