There are a few possibilities for this one. The first is that “they,” whoever they are, assume Captain Murphy is evil because of his deep voice. And they can’t look past that at all.
A second possibility would only be valid if Captain Murphy is in fact a new alter ego of some other rapper. In this case he would be essentially saying that his audience can’t figure out who he is because of his deep voice. Adding in a maniacal laugh just to poke a little fun at us.
A third possibility is that this line is preemptively trolling everyone for thinking Tyler is the Captain… if you listen really closely, his inflections don’t really sound like Tyler’s, and the voice is obviously deepened by effects but it’s not that much deeper than Tyler’s normal rapping voice (throughout the verse, not this particular line).