Troll picks

Something that I've always struggled with understanding is: what is a troll pick and is it a reportable offense? Disclaimer: the topic is mainly supposed to cover ranked as it's far more competetive than normal games. Last game now I had a Tristana jungle in ranked, it seemed as though she had no intention of winning or helping the team. While the player was not intentionally feeding or griefing me and some other team members experienced it as trolling to a certain extent. When some of the team members told the player they intended to report him/her, he/she simply claimed that he/she was doing his/her best and that it was toxic to claim that he/she was trolling. Was this an instance of a reportable offense?

If I use the same logic, could I justify picking AD bruiser Soraka Top as long as I claimed I was doing my best?