It's been wobbly since Labour got into power I imagine.
Long before then.
The problem isn't so much the extent to which it serves as a Party cheerleader (I doubt many readers have a problem with that; it's a partisan paper, everyone knows that) but the way it's done (too crude, and often too patronising (ie; even for a tabloid) as well). A lot of their politics coverage is focused on stuff that
Mirror readers aren't much interested in and with not enough on the sort of stuff that they actually
are interested in (but this has been the case for a few decades now).
The moralistic tone taken over that is still at odds with the paper's readers.
It also has too much celebrity news and not enough sport. I think one problem is that successive editors and owners have got it into their heads that
Mirror readers want a sort of leftish
Sun.