It's definately supposed to be lumpy - I found it can be a bit thin as well as lumpy... ew!.. so I add the measured hot water in slowly whilst stirring to a paste with a fork - keeps the lumps to an oaty norm.
I also find leaving it for a couple of minutes when it's come out of the microwave for its 1 minute enables the porridge to thicken up a little - and you don't burn your tongue on it!