In any given jurisdiction where property is assessed for tax, that is the job of the duly appointed Assessor's Office (with some name variation, but the duty is the same). Based on what THEY assess the value of the property to be (which could be their estimate of the market value, but not always; not all places assess property the same way), the corresponding authority that levies taxes (such as the Treasurer) will then charge the property owner with whatever tax is owed based on the assessed value (times whatever tax rate is applicable for their jurisdiction).
For any of this to be relevant, there would have to be some allegation that Trump actually interfered with the assessment process by the city/county that resulted in his properties getting undervalued for taxation purposes.
But that's not what's been alleged, as far as I'm aware.
She's alleging that he "defrauded" the banks by inflating his property values to get higher lines of credit against his equity. You might be right that she's comparing the assessed value of his property for tax purposes to what he put down on loan applications.
Trump says that nobody was defrauded, and nobody was harmed because the loans were paid back in full, with interest.
This simple appeal to a jury should be all that Trump's team needs to make to put the whole thing in a context any jury would relate to:
Ladies and gentlemen of the jury, how many of you have seen the public television program Antiques Roadshow? How many times has the appraiser given a value "at auction" (presumably the market value) and then told the owner that for "insurance purposes" the value would be 20% higher?-PJWas that public television program encouraging its viewers to commit insurance fraud? Did anyone who actually insured their valuable item for higher than the appraised market value commit insurance fraud? Should the PBS network be indicted as a co-conspirator to commit insurance fraud?
The answer is obviously "No," and President Trump did not commit fraud against the bank either when he didn't use the county's appraised tax market value.
In addition to the political side of this, I'll bet the State of New York is making one last attempt to squeeze some money out of Trump because they hate him for moving to Florida.