I wouldn't say so. I've never been to Mazatlán, but I've heard it's quite nice.
IMO there aren't "bad cities" in Mexico, but cities might have neighborhoods or areas to avoid. I know people in Culiacán, famously at the center of the Sinaloa Cartel and often in the news as a dangerous place. They live a tranquil life, like anyone else would. What they complain most about is just how hot it gets in the summer.
Going back to my point though if a friend were visiting I'd just tell them to stick to Mazatlán or other known touristy places. Both for the peace of mind and because its easier to get around with English.
