probably you accepted the mission when you were 68, so the reward is stuck at that level.
Next time, don’t accept a mission until you’re 72 if you want the reward to be 72.
If you want that Love Thumper at 72 now, you’ll either have to trade for it or reset your entire UVHM play-through. But then you’ll be able to get everything at max level (and won’t lose any of your existing gear or levels).
Well. Seems that it was on 68 because of the previous mission.
That makes sense: the LT side quest is automatically unlocked by completing and handing in “Stalker of Stalkers”, so it wouldn’t surprise me if the mission level was set then. Same thing happens with story missions: the level of the next one is set immediately you turn in the preceding one.
well, actually one gets Best Mother’s Day Ever from a box that stalkers drop after Stalker of Stalkers has been completed: one just has to ignore the box until the desired level has been reached.