Ok, so let's look at this in a bit of context.
Here's the seat gains relative to where in the country. Note Labour doing very well in making gains in London. Not so much elsewhere in country.
SauceIn terms of seats gained, and ignoring the context of which seats are being contested, this is a better performance than losing 18 (Corbyn in 2016) but about as good as Kinnock gaining 76 in 1988. [edit: was looking at earlier results, changed from 'not as good' to 'as good']. Of course, we're a long way short of Blair's 1800 odd in 1995. But so it goes. This is a bad metric to use for lots of reasons. But if anyone is really keen, fill your boots with 95.
Now it could be that Labour have so many councillors that they couldn't win more than 70 odd. That would, however, be a curious read of where things are generally at.
And would also need to ignore that the Tories made gains outside of London.
Against that, it's not actually a terrible result - if you look at it as a projected share of the vote (to smooth away the local election cycle where Labour are defending roughly 52% of the seats contested), then it's about par for a Labour government which has just won an election.
Of course, Labour didn't win the last election so make of that what you will.
What we have is a hugely unpopular government which after 8 years, a whole list of scandals and paraded nastiness, is on its way out with 35% of the vote. And opposing them is a wildly popular mass movement which is absolutely on course for winning the next general election by being the largest party. Coincidentally also on 35% of the vote.