Lee's definitely been reduced to a dimwit and it's probably meant to be down to his depression. The writers need to do better, though, because it's not being addressed directly and instead just making him irritating.
Storylines about mental illness generally conclude with the character going on medication and recovering (with little to no mention of it again until the writers want another dramatic meltdown), so it would be good to show how sometimes people can still struggle and require additional help or a different treatment. I hope that's where they're going, at least.