If you still have the patch in MQ you can do a side-by-side view using the ExtDiff extension, which you already have installed but probably not enabled.
Then you'd:
hg extdiff .... -r revision_before_patch -r revision_including_patch
On the other hand if you want to be able to do it from just original files and a .patch file you'll need to find a diff program that takes an original and a patch instead of an original and a result. My (old!) favorite is xxdiff.