I think the most important point to "Imasara Town" is Masara Town na koto. "What kind of a Pallet Town thing is that?" -> "What kinda noob talk is that?", since Pallet Town is the starting town in the 1st Gen games. The added "I" at the beginning seems to be a pun on the word 今更 (imasara), although I honestly don't get what that does here.
My Japanese blows, but you might be able to glean more complete information from these links:
Um...
SenpaiIsn't this a bit too short... What are you talking about, Imasara Town! Look at that veteran length
I dunno what Imasara Town is, maybe it's from ultra SM. Feel free to correct