Looks a bit funky without anti-aliasing because of the line thickness being unstable. You might want to try enabling AA as mentioned in this post.
A bug made it into 3.6 that can cause a crash when setting GraphicsDeviceManager.PreferMultiSampling to true because MonoGame internally tries to set the MS count too high, so you need to set the MultiSampleCount to a supported value like in the linked post for now. A fix will be in for 3.6.1.