Khan is by far one of the most well known political figures not actually in your parliament, he is in charge of your biggest and most important city. He goes out of his way to be in front of the cameras and he is the most likely "low" level functionary that will almost assuredly look to transition into a Parliamentary career. He also involves himself with policies involving the EU, and the UK in general, outside of the scope of his duties as a "simple mayor".
Khan may not be specifically responsible for what London currently is, but he is very much responsible for how it has evolved since taking the position. It's his policies that have seen increased surveillance. Its his policies that have seen the police services of London focus more on speech than actual crimes. Khan is really no better than one of those Evergreen teachers being given control of a major metropolitan center much like Trudeau is pretty much one given control of a country.
It's completely disingenuous to label Khan as nothing more than a simple mayor and you know that. He very likely gets more press than 90% of his political peers and superiors.