The standards required to play at Barcelona are probably the highest in the world and Mina is certainly going to face much better competition now. But in terms of media exposure and pressure, playing in a club like Palmeiras gives him a much better experience than playing in a mid-table european club.
This is an important factor and this experience should help him to adapt. Palmeiras is a huge club and the fans are particulary tough and demanding, not to mention the non-stop media coverage surrounding Rio de Janeiro and São Paulo clubs.
On the pitch, his passing is a bit rough at times, he will have to improve that, but he already has the physical presence and the instincts of a great defender. This is a great signing for Barcelona.
The only downside is that Arthur won´t come now.