Low? It shattered the records for concert films, becoming the highest grossing by a wide, wide margin. I really think your personal experience at the cinema might be coloring your judgment here. When you look purely at the facts, This Is It was a massive commercial and critical success. The people you were at the cinema with may have walked away from it thinking MJ was weird and crazy, but that definitely was not the consensus. When you look at reviews from the time, you see near-universal praise for his involvement in every detail of the show, his kind nature, and his ever-present talent (things we all knew as fans, but after years of negative press there were many people who never knew that or had forgotten).
Whatever your thoughts about the film, I think it did a ton of good for his reputation back then.