I had posted some of the recruiting rankings of the national champions since 2006 last year and went and found that post and updated it since then(added the 17 & 18 natl champs). I used the class rankings of the prior 5 classes and went to 2006 because there really isn't much for rankings on this site before 2002.
(Year)(Team) (last 5 class)(Avg per class)
2018 Clemson(8,22,6,4,13) Avg: 10.6
2017 Alabama(1,1,2,1,1)Avg: 1.2
2016 Clemson(6,4,13,14,14) Avg: 10.2
2015 Alabama(2,1,1,1,1) Avg: 1.2
2014 Ohio State(3,2,4,11,25) Avg: 9
2013 Florida State(10,6,2,10,7) Avg: 7
2012 Alabama(1,1,5,1,1) Avg: 1.8
2011 Alabama(1,5,1,1,10) Avg: 3.6
2010 Auburn(4,19,20,7,10) Avg: 12
2009 Alabama(1,1,10,11,18) Avg: 8.2
2008 Florida(3,1,2,15,7) Avg: 5.6
2007 LSU(4,7,22,2,1) Avg: 7.2
2006 Florida(2,15,7,2,20)9.2
National Playoff teams
2014(Average 8.6)
Oregon(26,22,16,9,13) Avg 17.2
Ohio St(3,2,4,11,25) Avg: 9
FSU(4,10,6,2,10) Avg: 6.4
Bama(1,1,1,1,5) Avg: 1.8
2015(Average ranking 14.2)
Clemson(4,13,14,14,8) Avg: 10.6
Bama(2,1,1,1,1) Avg: 1.2
OU(14,15,15,11,14) Avg: 13.8
Mich St(22,22,40,41,31) Avg 31.2
2016(Average Ranking 11)
Clemson(6,4,13,14,14) Avg: 10.2
Bama(1,2,1,1,1) Avg 1.2
Ohio St(3,9,3,2,4) Avg 4.2
Washington(37,30,36,18,21) Avg: 28.4
2017(Average Ranking 7.7)
OU(7,16,14,15,15) Avg: 10.4
Clemson(22,6,4,13,14) Avg: 11.8
Georgia(3,9,6,7,12) Avg: 7.4
Bama(1,1,2,1,1) Avg: 1.2
2018(Average Ranking 9.2)
Clemson(8,22,6,4,13)
Bama(7,1,1,2,1)
Notre Dame(11,13,13,11,11)
Oklahoma(8,7,16,14,15)
Obviously teams like Michigan State and Washington proved that you can get to the playoff without having a top 15 class, but it is a bit of an anomaly. Every national champion since 2007 has had at least 1 top 15 class and most of them have had multiple top 15 classes. Obviously you need the jimmies and the joes, but you need the right coach to lead them too.