Fiora's original niche is "the grand duelist". Theoretically the best champion in the game at dueling (which has never been true unfortunately), her main purpose since the rework has been a dedicated split pusher. Her teamfight presence is meh (the idea of the team-wide heal doesn't work very frequently, since pulling off all 4 vitals in a teamfight isn't fiesable in most circumstances) and her kit is almost entirely invalidated unless you're dueling.
The issue is, she gets slaughtered by several other duelists. Some champions she can have reasonable skill matchups with: Riven, Renekton, Darius, etc. A skilled fiora can outduel them effectively. But against other dedicated split-pushers, fiora gets slaughtered. Trynd, Camille and Quinn will crush fiora in a duel (particularly Camille has an incredibly similar niche but does it much better).
A lot of this can be attributed to some confusing design choices in her kit. The team-wide heal, for example, makes very little sense. Pulling off all 4 vitals in a teamfight is a bit crazy, and so much of that power is shunted out of dueling and into supporting her team. Putting %of max health true damage on her passive also adds some confusion to her kit. It is invariably a tank-killing stat, but because of this, the somewhat squishy duelests she fights aren't terribly affected by the strength of the passive.
At the moment, Fio's sitting in a tight spot. She's rarely ever used in soloque and her win rates are low. But what's more, she just feels very outclassed by other duelists. I'd love to see Fio be a champ that can win a duel vs any other champ if you've got the skill, but at the moment, her kit simply feels too disjointed to do so.