I can’t justify the supposed humor, but illegal Mexicans take menial jobs where they can get paid in cash and not have to provide documentation. Kinda hard to be an accountant, though I suppose he could be a motorcycle mechanic. But the joke relies on common expectation. Illegal Mexican gardener is common concept, whereas illegal Mexican motorcycle mechanic is a “wait, what?” moment. You have to spend too much brain power on the second part for the first part to have a chance at being funny.
Illegal Mexican gardener or landscaper is a known thing. Illegal Mexican motorcyle mechanic is not. Any humor is contrasting Mr. Law and Order Captain American with having anything illegal. Plus, he’s Captain America, why would he have a Mexican?
I figured the Captain America joke was playing on nationalistic anti-immigration teabagger types - you know, 'Murikins - who employ cheap illegal labor.
Since the early 1970s, Captain America has been written to reflect the political sensibilities of whichever scriptwriter was assigned. He (and sometimes she) may believe them to be quintessentially American, but they’re really pretty individualized, making the character, somewhat conversely, a “Man without a Country.” This joke carries on the helter-skelter tradition.
She spent probably two decades (real world time, not Marvel time) permanently hulked out and unable to transform back, so for that time all her lawyering had to be big, green, and hot.