I think the Mayor of London has been a personality cult from the start, with Ken Livingstone's photo on everything in the early years. TfL is naturally one area where the mayor can make their presence felt, and they've all made the most of it
Perhaps that's inevitable from the way that the Mayor+GLA system was deliberately established, revolving around one person with most of the power. If you do that, rather than having a traditional council set-up with more sharing of power and portfolios, then this is the result. Hence the annoyance amongst many Londoners when the GLC - as abolished by Thatcher - wasn't resurrected under Blair, but instead replaced by the system we currently have. (The referendum to do this was originally going to ask the two questions - re the Mayor and the Assembly - separately; but when it became clear that there would be lots of split votes [mainly pro-Assembly anti-Mayor, but with some Tories calling for
vice versa] it was changed to have just one vote for the whole plan. Which resulted in a lot of deliberately spoilt papers!)
The same criticism applies to many other local authorities around the country with all the power in the hands of one person.