The purpose of "whataboutism" is to point out hypocrisy in your debate opponent's position.
No, it is not. It would be if the argument was, for example, "which candidate is better" or "who should I vote for". But that wouldn't be "whataboutism" either, it would be just "point out hypocrisy".
If we are talking about just that single person (not even in a political way) and you bring up someone else just to deviate the attention, that is whataboutism and it's poison for the mind.
Rather than respond to that and have a reasonable conversation...
People that use this mechanism don't whant to have a "reasonable conversation", they just want to be right at all cost, even by sabotaging the debate. If you want to engage with them feel free to waste your time. I value mine more than that.
Plus keeping the argument going will make relationships worse: I voluntarely crash arguments like that with my father because yes, I do think that he's a dumb cunt, but at the end of the day I still want to say him "I love you nonetheless".