Not average-average, but in that tournament, yes they are.
Firstly, you are presuming that this will definitely happen.
Look at Cilic at the 2014 US Open, it didn't happen then.
Look at Stan at the 2014 Australian Open, it didn't happen then.
Look at Stan at the 2015 French Open, it didn't happen then.
Those are just from the last couple of years.
It didn't happen to Soderling at RG 2009 (at least until the final), he still smashed his opponents in the three rounds subsequent to the Nadal match.
It didn't happen to Gonzalez either, he played the subsequent SF lights-out and wasn't bad against Peak Federer either.
Secondly, if it does, it is not Federer's (or whoever's) fault. We cannot take anything away from them for it.
If an upset occurs, it is either because the top player was in sufficiently poor form for it to happen (so they wouldn't be beating Federer anyway if they were losing to Rosol for example), or the upsetting player was in sufficiently great form for it to happen.
You simply can't say that the victor of a match, in that form, is a worse opponent than the loser of a match (in that form).
There is no objectivity to that.
As I said to another poster just before, we don't know that at all.
The only evidence of their current form we have is a loss to a lower-ranked player, their other results (rolled into ranking) have little bearing on the tournament.
When you go out there, you do not have your name, you only have your racquet and your mind.
Djokovic in 2011 had won 3/4 slams going into the indoor season, yet he was playing awfully for most of it.
AO 2004 - Safin (former World #1)
RG 2004 - Lost to Kuerten (former World #1 and three-time RG champion)
WIM 2004 - Roddick (World #2)
USO 2004 - Hewitt (World #4)
AO 2005 - Lost to Safin (World #4 and former World #1)
RG 2005 - Lost to Nadal (World #4)
WIM 2005 - Roddick (World #2)
USO 2005 - Agassi (World #7 and two-time USO champ), also beat World #3 Hewitt in the SF
AO 2006 - Baghdatis (beat World #5 Davydenko in QF though)
RG 2006 - Lost to Nadal (World #2 and defending champion)
WIM 2006 - Nadal (World #2)
USO 2006 - Roddick (World #9), also beat World #5 Blake in the QF and World #7 Davydenko in the SF
AO 2007 - Gonzalez (World #10), also beat World #6 Roddick in the SF and World #7 Robredo in the QF
RG 2007 - Lost to Nadal (World #2 and two-time defending champion)
WIM 2007 - Nadal (World #2)
USO 2007 - Djokovic (World #3), also beat World #4 Davydenko in the SF and World #5 Roddick in the QF
So during his prime years, two of his sixteen wins/losses at slams were "weak" losses by ranking (and one of then was a former World #1 who was back to form).
I didn't actually expect that to be honest (had never really searched those stats before), but I'm afraid that argument is in ruins.

All of those bar the two bolded were at least as good as anything Nadal or Djokovic have had to go through in terms of ranking of opponents.
And even the two "weak" losses were because those players were on fire (and were still good in the finals against Federer too).
How "inferior" they "normally" are is irrelevant I'm afraid.
That didn't help their opponents in the tournament in question, did it?
Look above.
That "weak competition" and those "easy draws" practically didn't exist.
They never did, it's a myth.
In 14/16 of his prime tournaments, he faced serious competition (if you want to go off rankings at least).