Pioli's men went into the interval at Stadio Artemio Franchi 2-1 up courtesy of strikes from Ivan Perisic and Mauro Icardi.
However, despite Federico Bernardeschi missing a penalty, Fiorentina raced into a 5-2 lead by the 79th minute, and though two late efforts from Icardi applied some pressure in the closing stages, Inter slumped to a third defeat in their last four Serie A games.
And Pioli - whose post-match comments were delayed after he remained behind with his team in the away changing room - held nothing back in his criticism.
"It was the worst performance of the season. We were unable to react to difficulties," Pioli told Mediaset Premium.
"We have five games to regain the credibility that we threw away. We have hit rock bottom in the second half.
"We have to do the assessments and analysis to understand what happened in the second half, because that cannot be my team. We had an unexplained blackout."
Fiorentina's victory lifts them onto 55 points, one behind Inter, who are seventh in the table.