Indeed. If you're genuinely a bad therapist, but fool yourself into thinking that's not the case, then doesn't that mean you could end up worsening a bunch of people's lives? It depends on exactly what "bad" means—if it's "takes longer to help narrow down the causes of someone's problems", then that's probably not too bad, but if it's "subtly but consistently exacerbates one's patients' problems" or "recommends hallucinogenic drugs to patients with a family history of schizophrenia" (I've heard this is likely to trigger permanent issues; some googling gives conflicted but mostly supporting results), then you could really mess people up.

And if you know what your flaws are, then you can usually limit their impact. (For example, if you know you're bad at figuring out the right thing to recommend—either due to not knowing the field or being bad at learning about the patient's traits or situational details that make some approaches good and others inappropriate—then you could make fewer recommendations and/or couch them with more of the "here are a few possibilities that come to mind, I don't know which if any might work well, if you do try them pay careful attention and reevaluate" than you otherwise would; or consult better therapists and/or refer patients to them more often.) That may translate directly into becoming a "better" therapist, I guess, although you may still not be "good".