@Andrew was quicker, so I will just post this in case it offers something supplementary.
Officially, without any connotation of irony, they are called street preachers. Wikipedia explains that:
Open-air preaching, street preaching, or public preaching is the act of evangelizing a religious faith in public places.
However, this endeavour has lost its popularity, attractiveness and efficiency due to factors which I will not go into in this post. This Christian site shows how the name of what was of old an honourable means to spread good, has slid towards negative connotations:
Street preaching, or preaching openly in a public area, has been a method used throughout the history of Christianity for the purpose of evangelizing people who would not typically enter a church. Ever since the apostle Peter preached in the streets of Jerusalem in Acts 2, Christians have used this method to lead many to faith in Christ.
Despite the long-standing tradition of street preaching throughout church history, some believe that the practice should no longer be used. Critics believe street preaching has lost effectiveness as compared to its results in past decades. Second, some believe that street preaching is too overt or offensive, that people are turned away rather than drawn to Christ. Third, some critics note that certain people have used the “soapbox” to spread extremism, political wrangling or bad theology, giving street preaching a negative association. As a result, they argue, Christians should use other forms of outreach. (gotquestions.org)
I have underlined the word soapbox, because it can be used ironically in connection with street preachers, although it is used for political speeches mainly, as those you would hear in London's Hyde Park Corner.
I know Urban Dictionary is not a favourite here, but I will risk it since it does address the meaning you describe:
Street preacher aka street prankster (alternative name)
They can often be found on street corners talking freaky stuff about the Lord and repentance and amusing, sometimes annoying, the general public.
Under the same entry you will find a reference about the Welsh music band Manic Street Preachers. :)