Why "whataboutism" is bad
Pointing at someone else does not absolve bad behavior
Moral standards are universal, and are not dependent on what someone else does. They are certainly not dependent on what a political opponent does. This is why we need to stop relying on "whataboutism" in our discussions and analysis of politics and culture. We need to address arguments and criticisms directly, rather than trying to distract from the topic.
Whataboutism is when someone is faced with a difficult problem or argument, and attempts to shift focus to something else. When a public figure or group does X, we too often shift focus to something else. "Well, someone on your side also did/said X, so what about that?" But the question is not whether others did X. The question is whether X is bad, or whether it can be justified and defended.
For example, if Politician 1 (P1) was caught lying, we should not attempt to distract from that by pointing out that Politician 2 (P2) has also been caught lying. We should address that directly. Lying is bad, and we should not try to distract from P1's lies. We should acknowledge and agree that P1 should not lie and that everyone should be truthful.
Note that this does not prohibit pointing out hypocrisy and double standards. We can acknowledge that lying is bad, while also pointing out that the people criticizing P1 had no problem with P2 engaging in the same behavior. P2's supporters, then, have no problem with lying unless it can be used to politically damage P1.
It is also important that we have equal (or reasonably equal) sanctions for bad behavior. We see this commonly referenced in the phrase "for my friends, everything. For my enemies, the law." If we have criminal behavior by both sides of a controversy, but one side is harshly punished while the other side is not prosecuted or gets a light punishment, it destroys respect for the rule of law and undermines the moral authority of and trust in the criminal justice system. For our system to work, we need to have confidence that everyone is treated fairly and one group is not getting special privileges.
Ultimately, though, it all comes down to intellectual honesty. Exposing hypocrisy is fine as long as we are not doing it to justify or minimize bad behavior by people we support and defend. Partisan loyalty should not compromise principle or moral standards.