Is it wrong to build a wall on the southern border of the United States?

Hell no. It's the only solution to rampant illegal immigration.

the canadians probably wish we'd do it on the northern border too

No, just expensive and impractical. Consider how many got through the carefully built very high and thick extremely well guarded Berlin Wall. We need to change our immigration laws and policies and crack down on all the giant corporations who benefit from hiring illegals.

Nope it would help

No, just ineffective. See China, Great Wall of. Berlin Wall was only slightly effective because of the limited area it needed to cover, but people who wanted to cross found a way still.

It would be dumb

Don't you guys have fence already?

there already is one

Yes