Whataboutism is a fallacy in argument hinging on the accusation of hypocrisy.
It goes back to Cold War attempts to refute accusations about communist evil by pointing out that the west committed evils as well. It would not be classic example of Whataboutism to bring up America's negatives in response to a discussion of its positives.
Whataboutism would be hard to apply in the context of someone defending Trump. If you were attacking Biden's economic policies, Whataboutism would be to point out that Trump's economic policies are bad too--not as a relevant data point in the explicit comparison of their policies, but as a supposed refutation of the mere claim about Biden's economic policies were bad. So if you said, you weren't looking forward to Biden's deficit spending, it would be whataboutism to say your assertion is invalid simply because Trump has also had deficit spending. If the discussion were about the overall policy legacies of Trump (or Biden, or the USSR), it's not an accusation of hypocrisy to urge a look at both positives and negatives.
In my opinion, whataboutism now refers generally to changing the subject.
One of the great casualties of our time is the misuse of this term, whataboutism.
I miss the more precise and narrow meaning of whataboutism.