This is so because you are dealing here with the name of a function ; a great many compound nouns are constructed that way. This is not only true for the functions that people can take on but also for things. In some cases, corresponding to the compound name of a function, you find also the possibility to build a compound in which the title of the function is applied to a particular person or thing and a particular element; this is the case for “maitresse de maison”. This correspondence is only a semi-productive (ref.) feature of the French language; that is, you must learn on a case by case basis whether it exists or not.

no correspondence

• maitre d’école
• maitre de chant
• chef de gare
• chef d’orchestre
• inspecteur de police
• agent de change
• joueur de violon (people would rather say “violoniste”, (ref))
• pilote de ligne
• char d’assault
• voiture de course

correspondence

• maitresse de maison → maitresse de la maison
• chef de section → chef de la section
• directeur d’école → directeur de l’école
• voiture d’enfant → voiture de l’enfant

Note Since the 1990 orthographic reform law, you do not have to write “maîtresse” any more although it is still not wrong to do so for the time being; “maitresse” is also correct now.

You need to add more context because both are, and can, be used.

As far as the definition goes this is what we can find :

♦ Maître, maîtresse de (la) maison. Celui, celle qui dirige la maison, et en particulier, assure la réception des hôtes.^* (TLF)

Since you are not giving any context I will go through it with examples I have found.

• In the following example the words are included in a series of general concepts, no definite article since the author has no particular house in mind.

  Ainsi, une femme est née pour être une femme à la mode, une charmante maîtresse de maison, comme un homme est né général ou poète. (Balzac, Le Contrat de Mariage).

• The book L’encyclopédie de la maitresse de maison also uses the word as a general concept.

• In the following example the author is talking about a particular house. She could have used maitresse de maison, without the definite article, if she had wanted to refer to a particular house.

  La véritable maîtresse de la maison, quant à l’aspect et au maintien, c’est vous, ma chère Pauline, (George Sand, Césarine Dietrich)

Maitresse (maitre) de maison : we deal with the general concept, with no particlar house in mind → no definite article.

Maitresse (maitre) de la maison : we have a particular house in mind → we use the definite article.

_{^* My emphasis.}

“maîtresse de maison” is a skill: it is true anywhere, not only at home.

Whatever she has this skill or not, at home she is the “maîtresse des lieux” :^)