Ha, it can seem like that. Yes, only about 1/3 of the grinding is actually level grinding. In Disgaea 4 you can actually go from level 1 to 9999 in one battle, so levels are pretty much pointless beyond Reincarnation for the Character World and the aptitude bonuses you get on story characters for their first five reincarnations.
The rest of it is item level grinding. I'm not sure if you've done so yet, but make a "runner." This is a character with high move who can float, usually, and get past enemies who block the path. People like the Masked Heroes, I just use my Thief. Don't bother killing everything on every floor and you can get through 100 floors of the item world relatively quickly.
This is how you know someone didn't get into the post-game meat: if they still complain about how much level grinding there is when they should be complaining about item grinding.
Disgaea 4 also introduced some obnoxious grinding for ship parts. Have fun with the Kitty ship.