I think you mean tax minimisation. Nobody optimises their taxes deliberately.
It's not a crime, that's why the choice of words is so careful. BUT it's a very bad look for a Presidential candidate and a sitting President.
But your use of a convenient term ignores the serious possibility of outright fraud if the claims that Trump falsely declared the value of his properties in different contexts.
If he stated the value was high, when seeking loans that would be fraud. If he stated the value was low, when seeking lower tax, that would be fraud. That would be illegal in each case, if proven.
