Honestly with these things it depends who you ask.

The Mirror is a mid-level tabloid; not as bad as the Sun but not as good as a 'serious' paper like the Times, Guardian e.t.c. Some shitty papers still have decent sports coverage though, which is anyway all we care about here.

In my opinion the Mirror's football coverage is OK-ish, there are a couple of journalists there, John Cross and Oliver Holt seem to Know Their Shit. They've had a couple of decent Arsenal scoops in the last seasons, but they've printed some stuff which turned out to be bullshit too (pretty much all papers have).

Sorry if that's a bit vague, I really just think it's best to read a few different sources and for a particular story try to work out what the source is (are there any quotes? from where?) and use your own common sense whether it's plausible.