Current AQI in london ULEZ is 48 according to google maps which is not that good. Does that mean AQI is not a very good measure of air quality?

48 is decent - at the high end of the "good" range - but AQI fluctuates a lot from day to day. There were some fires on the other side of the continent from me and that was enough to bump the AQI here above 80.

Here's a report with some longer term trends (warning: 2MB PDF download): https://www.london.gov.uk/media/105046/download . Air pollution is down across London, and sharply so on the most proximate roadside sensors.

I don't know the current average, but it used to often be much higher than that. Maybe the average has improved?

It doesn't really make sense what you're saying. First you say it's not good but then you question the index. You're clearly using the AQI to base your opinion on? To answer the ULEZ question you should compare to not having ULEZ there, which is what the GP was talking about.