The world isn't black and white. Flexibility, including selective enforcement, is necessary in a just system.
But the history of selective enforcement strongly suggests that it does not usually lead to just results. It is often instead something that unaccountable officials find themselves easily able to exploit for questionable purposes.
For a notable example, witness how selective enforcement during the War on Drugs was used to justify mass incarceration of blacks, even though actual rates of drug usage were similar in black and white communities.
Yep, and while we fix that bad law we need judges to be able to say "I won't apply that" or "I won't sentence you to jail for this". That's kinda the point.
Allowing judges to not enforce bad laws turns them into unelected legislators. It's also worse from a corruption perspective because a single bought judge in the right place is much more cost effective than having to buy a new randomly selected jury at every trial.
By that measure every law is a bad law.
All laws are in some degree bad; perfect laws do not exist.
Some laws are useful and produce more good than harm in the concrete situation in which they exist.
Should laws be improved where possible? Yes. Does the need for selective enforcement indicate a problem? Yes. Does it provide sufficient information to determine the precise form of a better law to replace the one it shows a problem with? Very rarely.