It's not that hard. There are lots of objective criteria you can apply to show that one piece of code is better than an alternative: it's more efficient, it's more extensible, it's more reusable, it's less lines of code, it's less prone to error X when programmer Y tries to use it.
The hard part is actually convincing a human that something they did is sub-standard based on a set of objective criteria.
They're objective in that people agree on them. All programmers would say that a more efficient or shorter or more reusable piece of code is better, notwithstanding other tradeoffs that it makes.
My use of "objective" is to distinguish from subjective criteria that people often don't agree on, like "it's more idiomatic" or "it's simpler."
Whilst the measures may be objective, the ordering of them is not. That's quite subjective.
Personally, a shorter piece of code is only better if it maintains the clarity of the longer that it is replacing - assuming all things being equal. Code is always a trade-off.
The hard part is actually convincing a human that something they did is sub-standard based on a set of objective criteria.