He did pretty well in London - certainly better than Kahn, who has been disastrous.
Other than that, the views on London mayors tends to mostly follow party lines, so it's not a particularly worthwhile discussion.
And even if you had been right, the office of the mayor only has oversight powers and limited powers to set priorities and shift some resources. Most of the Met budget and control of their day to day operations is outside the mayors purview.
EDIT: Here's a wikipedia page with info on the media frenzy around the violence in London last year, and the comparisons to New York in particular, which basically jumped to lots of conclusions from a single month: https://en.wikipedia.org/wiki/Crime_in_London
And here is stats for April, that shows a higher rate of violent crime in Sussex (233 incidents per 100,000 people) than in London (227 per 100,000 people): https://ukcrimestats.com/Police_Forces/ - however this too means pretty much nothing; the numbers fluctuate too much to start drawing many conclusions based on the headline numbers.
But I guess by your argument there's a "bloobath" going on in Sussex then.
Just a laughable assertion.
Johnson's time as mayor was marked by trying to claim the successes of others (buses and bikes introduced by Livingstone), and failed initiatives like the expensive garden bridge which was never built or buying illegal water canons which could not be used, and addressing non-problems like drinking on the tube.
Khan's London has been professionally run and is tackling real problems like air quality and too many cars.
Both suffered from problems with crime caused by a decade of police cuts imposed by central government. London is still dramatically safer than any large US city though, including New York.