Being direct & honest isn't in itself a sign on mental illness, HOW you express it definitely can. Blurting out whatever one thinks without a filter if being tactful is definitely a sign of a problem.....thinking the thoughts through & expressing them in a thoughtful way shows emotional maturity.

HOWEVER .....in my marriage, I had lost ALL tolerance & was pushed to the point I didn't care how I said what I said before I was finally able to leave. I think in a relationship that is strained anyway there can come a time when people don't care how they say what they have to say.....usually called "burning bridges"

So there is no actual one set answer. It really does depend on the actual situation/relationship involved in.