I'd call it
mockney myself!
But it clearly makes it harder to understand the further removed from the dialect you are. When Planescape first came out I lived in Manchester, and I found it hard work to decipher. Having lived near London for the last 20+ years, I now find it quite easy. I can see why London was used as the model for Sigil - it's like a supermassive black hole sucking everything in, and with a gravity that is impossible to escape. But dialect gets even harder to understand for readers with English as their second language, which is a significant market for WotC. And it's very difficult to translate - you have to try to pick an equivalent dialect in the target language. So dropping or downplaying it seems like a sound commercial decision.
"Everyone in fantasyland has a British accent" is one of my least favourite tropes anyway.