What are the signs of failed DBT therapist?
Meaning what are the signs of not good DBT therapist?

[here today]

I'd say that a client feeling like the therapist failed them is a pretty good sign.

Other than that, I don't know of anything specific for a DBT therapist but here is one list of warning signs about questionable therapy in general.

50 Warning Signs of Questionable Therapy and Counseling

[TicTacGo]

Just like @here today said,

I do not know the context of the question, but I agree that sometimes the "feeling" that something is not right can be a good indication, regardless of the specific area such as DBT, whether it be regarding progress in therapy, ethical dilemmas or how they generally approach therapy and the topics brought up.

Something I learned from reading articles, watching videos etc. (all by therapists/counselors) is that the client should like their therapist. Yes, certain topics may be uncomfortable or difficult for the client to speak about, but one should generally like their therapist/counselor/psychiatrist, and not get that "feeling" of not liking/being comfortable with how they, the clinician, speaks to them, about topics brought up and so forth,

If I had to say what I'd want to experience with a therapist, it would be a feeling of having a safe place to talk about things without judgment and actually experience some improvement over time (as this does not happen immediately).

Best of luck,