Property taxes are generally based on the value of the house as determined by the tax assessor at the time that value is reassessed. Different places have different laws and rules that govern how the assessor determines the value of the house, but, as the article says, since the assessment is done by a human being, there will always be some subjectivity in the outcome.
The reason we the concept of "curb appeal" exists is that the market shows that houses that look nice and/or fancy sell for more than identical houses that are ugly or messy. So, it's logical that, unless there were laws or regulations that prevented this, a pretty looking house would be judged to be worth more than an ugly house – and, therefore, face a correspondingly higher tax bill when reassessed.
So, there's no "beauty tax" – it's just a side effect of pegging property taxes to value and the effect of "curb appeal" on property value