Is it a problem if a race/class is popular, with a high degree of satisfaction, but mechanically weaker than less popular options? Making the cool race/class more mechanically equal will result in it being even more popular and over-represented.
Yes and no.
Firstly, don't conflate "high degree of satisfaction" with "popular." They're very different things. Something can be extremely satisfying for the small slice of people who make use of it, while being almost totally ignored or avoided by everyone else. Conversely, something can be extremely popular but not very satisfying; consider a fair amount of fast food out there.
What we end up with is three variables: "popularity" (how common it is to see the option in use), "satisfaction" (how happy users are with their choice), and "strength" (how effective the option is in practice.) The ideal, of course, is to have average to high popularity, high satisfaction, and average strength. Humans (in both WoW and D&D), and blood elves (in WoW), are
extremely high in popularity to the point that they crowd out other options, middling in terms of satisfaction, and low in terms of strength. That's not the absolute worst situation to be in (the worst would be sky-high popularity but rock-bottom satisfaction and imbalanced strength, whether dramatically too
low or too
high), but it isn't a good situation to be in.
You cannot prevent some options ending up being more popular than others; that will always happen. But, by making different options have similar strength, you allow players to make their choices based on what they
like, rather than what they
feel they must choose. That, in general, is a better state of affairs than one where players feel "forced" into doing something. When players make value-judgments rather than calculations, they will generally be happier and more likely to stay engaged.
Plus, increasing the power level of humans slightly (so they're "average" instead of "slightly weak," which it looks like 5.1e is going to do) isn't going to meaningfully affect the presence of other races. The gravitational pull of "play a human, aka the species all of us actually are" predominates over any other concerns, such that
if a player is already willing to play some other option to begin with, it
generally won't be because they wanted power. Doubly so because, as noted, it is only
standard human that is weak;
variant human is as strong as the strongest feats, which makes it strong indeed. Particularly if you're in a game where feats are otherwise inaccessible.