Good question. Both are grammatically correct, but they sound different in English. “Boss lady” is a casual, modern phrase. People often use it playfully to mean a strong or confident woman in charge. “Lady boss” is also possible, but it sounds old-fashioned and less common. Most native speakers today would say “boss lady.” So if you want to sound natural in everyday English, “boss lady” is the better choice.

They are both correct, but with slightly different meanings because the noun and adjective are switched. "Boss lady" means a lady who is [a] boss. I think the adjective 'boss' used here is not necessarily the narrower meaning of 'person in charge within an organisational structure', but a broader meaning of 'excellent, ambitious, successful, in control'. You might say "she's such a boss lady" to express admiration for someone. "Lady boss" means a boss who is a lady, it's used to specify the gender of the boss or to refer to the wife of a business owner. I agree with Abby that "boss lady" is a much more modern phrase, though I think it might be more common in American English - here in Australia I don't think I've ever heard it being used. Whereas "lady boss" is something that I commonly hear and use, particularly in Asian-owned businesses.

I think "lady boss" is just normally to mention that the boss in any organisation or something like that is a lady and "boss lady" directly tells about a strong and confident lady.