Basing on statistics, nowadays the number of Mexican immigrants in the USA has reduced a lot compared to 15 years ago. But still in Donald Trump's opinion immigration is a very big problem.
To my mind what Trump states is something untrue and wich can't be considered such a big trouble to cause the construction of a wall between two countries. Infact we must consider that the USA in the last years have welcomed less people than Italy, France, Austria, Sweden, Germany and Hungery.
We must also consider the enormous expence of the construction of the wall and that he promised this wall during the election campaign and he can't now go back to his words so I think that , even though Trump doesn't probably considers the necessity of a wall, he has to start up with it just because he wants to be faithful to his promises not considering what it will cause from a social, political and economic point of view.