It's not about his actual mum. Thanks for that.
But I think the narrative of the song is more or less literal, although used to explore a number of ideas. I think the subject of the song is an interesting continuation of a theme that's persisted throughout his work - it instantly reminded me of 'Mother, I can feel the soil falling over my head.' There's always been an association between Mother and Suffocation/Death ('Smother me mother'), hated and embraced. Remember he always 'ran back to Ma' on all his albums.
People have complained about a lack of empathy but this lyric is an expression of really powerful love and sympathy for someone who has been driven to exhaustion by a world unsuited to them (see Oscar Wilde, James Dean) but the song's narrator feels an inescapable, suffocating affinity with, and a rash, childish-adolescant, protective, passionate Hand-in-Glove love for ('I will slit their throats for you'). I think the end of the song is a fine example of that old Morrissey trick of making what is intensely personal, universal, and creating that 'Shoplifers of the World' unity, seige-mentality (We're gonna run to you, we're gonna come to you) while at the same time making the filial, almost sexual: 'We're gonna lie down beside you Mama.'
As a whole, it seems like a half-remembered expression of a child's emotions, which is illustrated in the wording of the title, the past tense and the use of the diminutive 'Mama', but tinged with an acquired bitterness at the 'lonely' world he has experienced 'here without you', berating the 'pigs in grey suits' ('policewomen, policemen, silly women, taxmen'), perhaps expressing a projected anger for an absent yet idolised mother figure.
It's a f***ing great song.