If the opinion of Yoshi's viability can change on a dime then you have to admit that we may not fully understand the capabilities of ANY character. Characters such as Fox and Falco are extraordinarily well represented at high level play and draw from a larger pool of talented and motivated players who are able to innovate them. Writing off characters such as Sheik or Marth when they aren't being equally represented is not a reliable way of assessing who the best character is.
Honestly, Sheik's toolset is RIDICULOUS. She has versatile edgeguarding, fantastic projectiles, incredible normals, above average attack range, great ground speed, amazing ledge game, reliable set ups, godlike kill conversions, etc. If she isn't the best character in the game, then she is certainly the most effective and consistent. I feel like her strengths can be exploited further but the character doesn't seem particularly popular in modern meta. M2K, KK, and Ice are kind of alone in the trenches at top level.