Well, it's safe for you to come and visit Mexico City as a tourist, there are several tours that can guide you and you have nothing to fear. Just be careful with your belongings when you are surrounded by lots of people (like in the subway) and don't wander in lonely streets in the night.
About the weather, these days we are experiencing rain but it seems to be decreasing, and it must be sunny (occasional rains) with template weather the next weeks. about 9 years ago
I was in Mexico CIty over Christmas and it was safe during the day, but it did feel a bit creepy at night time approached, but that could have just been over cautious.
I suppose guards with sub machine guns at shopping centre entrances must be there for a reason about 9 years ago