Why the appearance of One Piece characters have goatee after time skip?

Question

After time skip, the appearance of characters in One Piece are changed. Several of them change significantly like Nami with her long hair, Robin with brighter skin, Chopper with all of his transformation different before time skip, even Usopp have more muscular body and longer hair, he also have goatee.

But I want to know what happened with this goatee. Beside Usopp, other characters also have goatee after time skip like :

Sanji

Capone Bege

Well maybe I'm not sure who's the others, but why many of them have goatee? First I think it's for make them more mature or to shows that time has passed several years, but when I see (maybe it's bad example) Pokemon, in the latest generation they have another "evolution" called mega evolution, in this mega form, many Pokemon changes their appearance with some goatee like attributes.

Is this goatee is trend in Japan or what?

Answer 1

I think it's for make them more mature or to shows that time has passed several years

Your way of thinking is correct here. Although it certainly makes them look mature, facial hair growth more correctly shows the passage of time. It is known as the TimePassageBeard trope.

Is this goatee is trend in Japan or what?

It's not a trend in Japan, but one of the tools/visual cues used by writers (and mangakas too) to easily show the passage of time. This kind of trope is not only seen in anime, but in other types of media as well. The previously linked page lists some examples, but let me put some here whilst adding some of my own:

1. In The Big Bang Theory, At the end of season two the gang goes to the North Pole for three months. When they come back in the following premiere they all have big beards (except Sheldon, who has a neatly-trimmed goatee).^{This came from the linked page}
2. In the recent movie In the Heart of the Sea, the main character Owen Chase is shown to have grown facial hair to show the passage of time.
3. In the game Assassin's Creed II, main character Ezio Auditore is shown to grow facial hair as you finish more memories.

So why goatees? Or more importantly why hair? The keyword here is easily show.

Passage of Time: Human hair growth speed is roughly 1.25 cm per month. That means that if you don't cut your hair (facial or not) for a year, it will grow at least 12 cm, the size of a ruler! The average human face is roughly 50 cm in length. Having hair that is 12 cm long in that face is easily visible. And because we all know that hair grows with time, seeing a long hair on someone helps establish that some time has passed since we last seen him/her.

Maturity: As for maturity, it's simple. In the ancient times and even in the modern times, facial hair is always seen in adult people. You don't see facial hair in young people right? Putting facial hair on a character helps establish the fact that this character isn't young anymore.

tldr; Yes you are correct. Goatees, or hair growth in general easily shows maturity and/or passage of time.

As for the Pokemon, at least in Alakazam's case, it looks like the goatee there more symbolizes wisdom than passage of time. As for others, well...