Hi, there is some clinical evidence that mindfulness is effective in treating depression in some but not all cases. https://www.evidence.nhs.uk/Search?q=mindfulness+depression

That doesn't mean it will work for everyone or that it is the best treatment for someone it will work for.

I expect it depends on the source of the depression.

I don't think the effectiveness is just about changing a thought-pattern. It seems to me that there are a couple of things:

1) Changing your relationship to your thoughts. 2) Changing your thought patterns in order to change the neurochemical structure of your brain: https://en.m.wikipedia.org/wiki/Neuroplasticity

It's a similar theory to CBT and other therapeutic approaches which are known to be effective in some cases.