Slight correction to you topic, you originally say a customer bullied the women wearing the Trump shirt, then later say it was the barista, who is an employee.
If we're talking about a customer, taking someone else's property and writing on it is technically vandalism, which is a crime, and is unacceptable (minor sure, but it's not like you want random strangers touching your food without your permission).
The barista is an employee, and while I don't understand what pleasure people get out of it, writing on the cup is apparently part of their job. I however question the wisdom of unnecessarily involving politics into your job regardless of what the customer is wearing or what their political views are.
That said, if you're wearing a Trump shirt, seriously, ironically, or comically. I don't understand being offended by "BUILD A WALL".
If you are wearing the Trump shirt as a serious Trump supporter, you support "building a wall".
If you are wearing the Trump shirt ironically, you find the concept of "building a wall" absurd and would see someone writing "build a wall" on your cup as an agreement to the absurdity.
If you are wearing the Trump shirt for comedy, someone saying "build a wall" to you meant they got the joke.
The only fool bigger than the person who knows it all is the person who argues with him. They're all complacent sheeple. Passion fights, but reason wins.