It’s not really a matter of being “negative” - but rather, first of all, it seems unnecessary - Bush is way down the list in most polls - Trump has several people who are more an immediate threat to him including Cruz and Rubio whom it would seem he should really be concentrating on - also, his responses are not directed to the issues or policies but are personal - “He’s a loser” and “he’s a low energy person” are the sorts of things I would expect to hear on a school playground, not in a serious political debate....
You sound sooo PC. There is no special Presidential way to speak. You are trying to control speech style that you do not like.
Attacks on Bush are meaningful because some voters could be swayed by the Bush lying attack ads and vote for one of the other candidates thus possibly affecting Trumps overall vote count. Bush is in the race and if he attacks he needs to be attacked back, not ignored. We have seen this before and it does not work.