If Serena was losing often in slams to Dementieva it would be pretty bad. However she never loses to Dementieva in slams. And make of it what you will but Serena has made it clear she doesnt make winning non slam events a huge priority for a long time now, whether she cares or not, she certainly doesnt make it as much a priority as she could or should. So all that said Dementieva has not done enough damage to Serena to even be that relevant from that respect either, even if we ignore her minimal relevance as a key player of this era. Which is a far cry from Federer and Nadal. Add to all that Serena is Federer's age so if he has been past his prime for years, then Serena probably is by now as well. Dementieva couldnt touch Serena on any surface, in any match, from 1999-2004 as well. So based on that it would appear even if Nadal were to have not proven being able to beat an absolute peak (not even just prime but peak) Federer in his own pre prime days, in slam matches only, on non clay surfaces only; Dementieva has not proven ability to beat Serena other than a past her prime Serena on non slam matches only. Again there is no parallel between the two "rivalries" if you will.
I am not sure why you even made the comparision of the two honestly, so feel free to enlighten me on that as well. The only thing I can think of is
you are trying to exclude clay losses, you are trying to exclude matches Federer is supposably past his prime (keep in mind he was only 26 and 27 years old when he lost Wimbledon and Aussie Open finals to Nadal anyway), and you are trying to exclude non slam matches on non clay surfaces that a pre prime Nadal played vs a prime Federer to boot.
So excluding clay, excluding post total dominance, and exclude non slam matches. And you are remembering me mentioning Serena's losses to Dementieva in smaller tournaments dont matter since she always beats her where is matters in the slams. Still what you seem to be narrowing with Federer vs Nadal is alot more than what I was saying with Serena and Dementieva initially which was ONLY overlooking non slam matches and nothing else, a single whammy vs a triple whammy.
And yes I think your most important head to heads are vs the other greatest players of your generation or people you are most directly compared to. In Federer and Nadal's case they are so far and away the greatest of this era it makes it even more significant. Sampras would lose worse if he had a losing head to head with Agassi or Becker, as oppose to having ones with Stich and Krajicek (who he barely played in slams anyway). Graf would look worse if she had a losing head to head with Seles or Navratilova, than if she had one with Jo Durie (which she does). People dont even really care much about Agassi's losing head to head with Courier, and Courier is closer to Agassi level than some of these other examples. However one looks at Agassi's head to heads vs Sampras, Lendl, Federer, Becker, and Edberg, all before Courier.