One doesn't have to underestimate this USC team to expect a comfortable ND win, we just have to know what to expect based on both teams' performances over a nearly full season.
USC has a possibly lame duck head coach that most were surprised it hired. It starts a true frosh QB who should be in high school. It lacks the big-play capability we're used to seeing, and the OL has trouble establishing the run and protecting the QB against good defenses. It has lost four of five games, only beating 2-8 Oregon State in the process. It just lost to a UCLA team that entered the game with a 2-8 record. After blowing a 14-0 halftime lead to Cal in the span of about 10 minutes a couple weeks ago, USC didn't even come close to scoring when it needed one point for the final 20 minutes of the game. Its best drive after halftime was 10 plays for 33 yards and a punt. For the half, it ran 31 plays for 30 yards and -2 points.
This USC team is barely top 50 and I'm assuming that rating is living off the two-month-old home victory over Washington State. The two best defenses it has faced - Utah and Cal - held USC to 205 and 277 yards, respectively. USC is 5-6 for a reason. Just imagine Notre Dame is getting ready to play a different 5-6 team whose history doesn't frighten you.