He doesn't use it as a direct excuse as much as a mitigation.
He has so far used
- anti-irish sentiment
- hair colour
- depression
But no I don't think he has ever said "I threw a chair at him because I'm depressed".
Mind you when people commit serious crime and try to use "diminished responsibility" they do the same.
They don't say I did it because....
They say I did it but.....