I'm not knowledgeable enough to know whether Schilling's inability to filter himself is an indicator of mental issues. There have been other Phillies over the years who can't seem to have an unuttered thought. Billy Wagner comes to mind, and Shane Victorino, who if I recall correctly said he had ADHD. (Of course Shane was so lovable he was a joy to listen to.)
It's unfortunate that in Schilling's case that it may be affecting his Hall of Fame chances. He isn't a no brainer, and it would only take a few voters to decide that his offensive statements make him unworthy of their vote to keep him below the threshold.
Actually, if mental issues are contributing to Schilling's diatribes, I find him more sympathetic even though I still don't want to hear them.