That's an interesting question, and a very good point. In that situation, especially in a ranked match, I would definitely agree that it was trolling and griefing. Not learning any skills just puts a champion at too much of a risk for losing trades, losing lane, losing teamfights, and losing the game. It would be one thing if it was in a normal match or if the player was grouped with friends who were all aware of it (like playing Ultimate Bravery).
But as far as just not using an ability? It's like when you have a bad Bard on your team. Would you rather him ult badly, and cost the team fights and objectives? Or just not use the ult? Me personally, I'd rather him just not use the ult rather than get me and everyone else on the team killed, and have us lose a dragon, baron or turret for it.
There's always going to be a difference in deliberately trolling your team, and being unskilled with the champion. If the way to win a League match is through obtaining objectives, and the player is actively working to take those objectives then they are trying to win. Riot doesn't punish people for being bad players. If she was making use of her primary abilities, managing her summoner spells, farming, taking inhibitors, dragons, barons and turrets, building appropriately AND not flaming or saying anything negative to anyone on the team, even the players who were feeding pretty badly, I'd say she was doing decently. Especially on someone like a Kalista, whose primary damage/burst doesn't come from her ult, it comes from her rend.
Out of curiosity, did you ever play AP Sion before his rework? Back then, not only was it common to not bother using his ult, but you weren't supposed to even level it up at 6. You didn't put a point in it until you absolutely had to because there was nothing else to put a point in.