Inzaghi has been very inconsistent at Lazio.
In 2016/17, his first season, Lazio finished 5th in a season with both Inter and Milan in a bad shape. In the following season they finished 5th, choking in the last games and losing CL qualification in the last match of the season against Inter. In 2018/19 they had a terrible season and they finished 8th. In 2019/20, the only season i think Inzaghi overachieved, they were fighting for the Scudetto with Juve till February but post-covid they had a total collapse and they finished behind Inter and Atalanta. In 2020/21, they had a totally mediocre season and they finished 6th.
I see nothing special in Inzaghi, he's very hot and cold and he would have been fired at Inter during his bad runs. He's exactly where he's supposed to be. That's his level. Lazio.