In this context, "bubble reputation" refers to how easily a person's reputation can be destroyed or ruined. Macbeth is worried about how he will be remembered after his death, and he fears that his reputation will be tarnished by his crimes. He feels vulnerable and fragile, and he knows that his reputation could easily burst like a bubble.
This is a common theme in Shakespeare's plays, and it is often explored through the character of the villain. Shakespeare's villains are often complex and sympathetic figures, and they often struggle with their own guilt and remorse. They may have been corrupted by power, or they may have been driven to madness by their circumstances, but they are still human beings, and they are still capable of feeling the sting of conscience.
In the case of Macbeth, his "bubble reputation" is ultimately shattered when he is defeated in battle by Macduff. Macduff, a loyal nobleman who has been driven into exile by Macbeth, returns to Scotland to avenge Duncan's death. He kills Macbeth in single combat, and he proclaims himself as the new king of Scotland.
Macbeth's death marks the end of his tyrannical reign, and it also serves as a warning to those who would seek to gain power by immoral means. Macbeth's "bubble reputation" is a reminder that power can be fleeting, and that the pursuit of it can lead to ruin.