That is an extremely stupid thing to say, this is just one by-election and to be honest Karachi is not that important when it comes to national electoral politics. Karachi only holds what, i think 17 seats out of a Parliament that is made up of 342 Seats.
PTI is expected to clean sweep the LB Elections in KPK, they are menacingly strong in Punjab and the only party that is going to give PML-N a fight. PML-N is aware of the threat PTI poses and that is why it is desperately trying to postpone the LB Elections. You simply cannot compare a National Party to a City Party. The real Prize is Punjab, and PTI will go into elections in Punjab with the argument that they competed against MQM head on in their home-ground while PML-N is only a Punjab Based Party.
Agreed, PTI needs to became more organized in Karachi or even Sindh as a whole. They need to develop their 'Tanzeemi Nizam', something similar to what they are building in Punjab right now. Their Organization in Lahore, my home city is very impressive and very strong. They barely had any hiccups in shutting down Lahore during the Dharna.