Certified GKH Gardening Expert
When the flowers drop, if they are not removed, they can be reduced to a slimy mess which could suffocate a lawn or cause fungal disease. However, unless the grass is shade tolerant, just being located under a tree can kill grass. And yes, any covering of leaves that gets matted or sits on grass for too long has the potential to kill the grass and attract unwanted pests. You should try to keep the area raked and free of leaves. The raked leaves can be composted or run over with a composting mower.
Here's an article about Jacarandas: https://www.gardeningknowhow.com/ornamental/trees/jacaranda/jacaranda-tree-information.htm