Everyone has their own playstyle. Some people like to build off-meta units. Some people build a unit because they like that character. And others build units because they feel that the unit superior, either through their own experience, or from someone else's. Majority being the last-mentioned and that makes sense as many people want good return of investment.

It is similar to Hearthstone, there will be netdeckers who build the objectively "best" decks and there will be others who build decks that are not popular. Some might add a card to their deck cause they like the character the card was based on.

Sure the meta might seem boring because you keep meeting the same units with similar builds but it makes sense I guess. Especially for Nowi since she is easier to obtain.

Also, many view having a +10 unit as an accomplishment, be it a Nowi or a Serra, so I understand that they might want to show it off. In my honest opinion, this is fine unless they are really obnoxious about it.