Guessing it is common for the state department to send a representative. Perhaps the Secretary of State since it’s an important neighbour. Since it’s campaign time, you could see others going if they believe it helps their reelection bid somehow ( though I don’t know how), but it would not be unusual if only the US ambassador to Mexico is present. I don’t know of any foreign politician that attend the US presidential inauguration.

I mean you have to assume they're talking about like the governor of Texas or something, but I've never heard of them attending before.