Can anyone see the downside of this against the general populace of the American people?
This is not risk free to America. Prices of some goods will go up. Instead of addressing the migration issue, the Mexico government may issue retaliatory tariffs on US goods.
The Mexican government is very corrupt, and does not care about its people the same way an American President should. The Mexican government has also dumped many of its problems into the US over many decades.
Mexico is not in any position to make demands from the USA. Its primary income is from remittances from workers in the USA. All the USA would have to do is put its foot on that hose of money and they would choke.
In
2017, remittances to Mexico were $28 billion, compared to a
GDP of $1,150 Billion ($1.15 trillion), or about 2.4% of the Mexican economy. According to a recent
New York Times article, Mexico exported ~$346 Billion in goods to the US last year, or 30% of their economy.
A tax on Remittances would slow the inflow of migrants into the US because migrants would be unable to send many times the local wage back home to support their families.
You are looking at the entire GDP, not just foreign income. I should have specified but I failed to do so. The original premise still stands as this is quite a dependence on the US.
https://en.wikipedia.org/wiki/Economy_of_Mexico#Remittances