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.

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.