There isn't really such a thing as a "good" primary source or a "bad" primary source. Primary sources are valuable, but each source comes with its own unique issues and problems. The main issue with getting all your information about Nazi foreign policy from Hitler's personal writings is that Hitler's personal writings tend to be very ideologically driven. By that I mean, he kinda rambles on about what he wants to do, but when push came to shove he choose to do something different for pragmatic reasons. Hitler often betrayed his own ideological underpinnings in order to help out Germany's position in the war.

So while its certainly a valuable resource, its more valuable in understanding what Hitler thought would happen and what Hitler wanted to happen with regards to the war and Germany's future. Its not so great at showing what actually happened. If you interested in Nazi foreign policy you're better to go for a secondary source like Hitler's Foreign Policy 1933-1939: The Road to World War II by Gerhard Weinberg (the historian who discovered and translated Hitler's second book).