Nobody's saying that touchdowns are ALL about the receiver's talent though. But it certainly plays a part.
Brandon Marshall suffered a RELATIVE touchdown drought in Miami (emphasis on term 'relative') but he still scored literally more than 4x more touchdowns than Brian Hartline did over the same time period. Marshall scored 9 touchdowns in 2010 and 2011. That includes 6 touchdowns in 2011 when the offense got an upgrade on Henne at quarterback in Matt Moore. Over that time period Hartline scored 2 touchdowns, 1 each year. No increase in touchdowns when Moore took over. No increase in touchdowns when Tannehill took over, either.
The fact of the matter is if you look at targets and catches from 2002 to 2012, you get 367 wide receivers that qualify with either 60+ catches (2002 to 2005) or 100+ targets (2006 to 2012). Only 2 of those 367 wide receivers had a lower touchdown percentage on their catches than Brian Hartline did in 2012. That's it. Torry Holt in Jacksonville the year before he retired, and Laveranues Coles in Washington when Pat Ramsey was throwing the football.
And maybe you could even forgive that, IF he had shown a tendency to stick the ball in the end zone in previous years (as had both Holt and Coles). But in 2011 he had 1 touchdown and in 2010 he had 1 touchdown.
You could also forgive it IF you've seen the kind of physical ability that suggests he can do this regularly, and circumstances just worked against him. But that's not the case either. He's weak after the catch so he's not going to bull his way in from inside the 5 yard line the way Rishard Matthews did in the preseason, or the way a Hakeem Nicks does. He's not a strong catcher of the football in 50/50 or physically challenged catch situations, so no jump balls happening there. When he does get behind a defense, he gets caught from behind unless it is a ridiculously blown coverage (literally, see the Cardinals game).
People don't realize that when you get high on mediocrity at skill positions, you are in effect banning playmakers from your team -- at that very position.