In your sentence it is better to say "She shows good leadership" or "She shows good leadership skills". However, the verb demonstrate might be a better choice in this context as it indicates less elements of showmanship. I suggest that you say instead "She demonstrates good leadership".

You need to be careful where you use articles. There are certain nouns that don't use them. These are called "mass" or "uncountable" nouns. Qualities tend to be these uncountable nouns. You can't count "leadership", you can't "charisma", you can't count "beauty" etc. These tend to be used without articles.

It would be better to use. "She shows good leadership skills." since being a good leader involves various skills in order for one to be called a good leader.

In my opinion, you will hear more people say typically in an informal setting, " She shows good leadership." However, I believe "she demonstrates great leadership" is the sentence that I would use. I hope this helps!

It sounds more natural to say "she shows good leadership." :)