The title is wrong, but not because of “Melech al tekalel”. First of all, Trump is not a king.
Second, “Melech al tekalel” is not a commandment, it’s good advice. The posuk continues that the reason not to curse the king is that no matter how careful you are, you are likely to be caught. “Ki off hashomayim yolich es hakol.” If you could be sure of not being caught then you could go right ahead and curse. Now in this country we have freedom of speech, so even if we had a king we could curse him safely. (Indeed, even in the UK and most other countries with a melech, one can nowadays curse him with impunity.) And we certainly incur no risks by cursing the President. So while I don’t agree that he deserves cursing, those who think he does should feel free. That way the rest of us will know how little value we should put on their opinions.