Usage is related to power but higher usage does not necessarily mean higher power. If (rainbow) SoFo was suddenly not used at all it would still retain its power. SoSe's higher usage than SoFo does not make it more powerful.
Although this is true, it is a good indicator. If 1 card is used a lot more than another, we must ask ourselves why. Is it because card A is too powerful? OR could it be that card B is not powerful enough. The other interesting thing we have to deal with is the concept that shards are rare. All that really needs to happen for 1 rare card to be used more than another is for it to have a spike for a couple days of usage in arena/pvp. Then, after that, the player base will be more saturated with that card than other rare cards. This could also be the reason.
How I prefer to use this data is like so....
If a card is underused, I tend to think it is underpowered. Its generally the only real reason. The only other reason is that it doesnt have any combos that synergize with it, or has few combos that do. Then more cards would be the answer. Generally, this isnt the case though.
If a card is overused, I first check to see if it fulfills a specific niche. Supernova is a good example of a niche card. Not many other cards can do what it does. If the card is a niche card, then perhaps its time to give players some variety. That doesnt mean nerfing the card, it just means giving more alternatives through card creation.
If the card is not a niche card, and is something like a dragon, I take a look at the element. Is there a reason that this element is using its dragon more than others? For example, if Ruby dragon was used more, I would look at cremation and understand. If the time dragon was used more, Id look at instosis and understand.
I would decide it would need nerfing if (keeping on the subject of dragons) the water dragon was used the most. Water doesnt have any deck/combo/quanta generation that makes its dragon particularly stand out. If this card was overused, it would simply be because of it having blatantly overpowered stats, so it would be worth the investment to stick in a good percentage of decks.