I owned a home in Mar Vista for 8 years. Mar Vista covers a very large area and some neighborhoods are safer than others. Mar Vista Hill, Mar Vista Knoll and Westdale Trousdale have expensive homes and are generally perceived as safer. The blocks adjacent to Venice Blvd. and south are a little are closer to traffic and commercial areas and tend to have more graffiti, etc. I believe every neighborhood in Mar Vista has improved but there are still small pockets with gang activity. There are two long standing rival gangs, the V13 and Culver City Boyz but I couldn't tell you what they are up to these days. You might contact the Pacific division of the LAPD to get crime statistics.
Check out the Wikipedia page
If you have further questions, please feel free to contact me.
Aaron Mirsky, Coldwell Banker
I'm 35 year resident of Santa Monica and Mar Vista