Downtown Cancun has always been a place that an unwitting tourist doesn't want to roam around at 3 in the morning. But every major city in the world has parts like that, there are certainly several here in the US. I wouldn't be worried at all, even if you do choose to venture off the resort, just use common sense; travel in a large group, don't break up and stick to the well traveled road.
There have been 127 murders YTD in central Florida, 100 in QR. You don't see a travel warning for Disney or people cancelling trips to orlando do you? Pure trump wall propaganda bullshit ... once they issue a travel warning for the swim up bar I'll start worrying....
For a little perspective you might Google what advisories other countries provide for travel to US see the following example: www.washingtonpost.com/amphtml/blogs/govbeat/wp/2013/11/14/16-american-cities-foreign-governments-warn-their-citizens-about/