That would be amusing, but it’s not how US judges usually impose fines so they aren’t going to start doing it now.

One argument against using net worth is that someone could claim a very low net worth and escape a heavy fine. After all, calculating net worth is not straightforward (note that the previously claimed net worth is not necessarily the current net worth). Of course, the judge could open an investigation into the actual net worth in order to determine the fine. But that would take time and now the original trial would be delayed - which is what Trump wants. Not only that, but (unlike a standardized fine) a determination of net worth would open multiple avenues to appeal, meaning further delay. So in most cases the process would likely be more trouble than it’s worth.