Maximum trim length is the longest that you want the case to be, so shorter is better.
Generally, information on cartridges have a "maximum trim length" and a "minimum trim to length". These are usually 0.010" apart.
So, when the case exceeds the maximum length, it needs to be trimmed. Most trim to the minimum so that they do not have to trim the case as frequently.
If you insist on crimping your 223 Remington, it is desirable to have all the cases at the same length. It makes for a more uniform crimp from case to case. But, crimping the 223 Remington is not necessary. I do not crimp my 223 Remington ammunition.
I measure my 233 Remington cases after each resizing. Any that are too long get trimmed. Any that are under the max length do not get trimmed.
Hope this helps.