Trump is not building a wall around the perimeter of Mexico. The wall will be built to prevent illegal entry into the country along the US-Mexico border.
Every government has an obligation to its citizens to protect the borders. Try getting into the USA without the proper paperwork via one of its airports. You'll be interrogated, locked up and dumped on the next plane back to where you came from.
What's the point of having such tight controls at airports if the southern border with Mexico has not existent entry controls. You secure your house by locking all doors when you're not in. You don't just barricade the front door and leave the all the other doors and windows wide open.
What Trump is doing is straightforward common sense. More jobs, better security, lower taxes, smaller government, less red tape. I can understand him being ridiculed if he cannot deliver on his promises. However to take him to task for his sound policies is insane. People seem to have lost their minds over his win.