I wouldn't call the Mezuzah an angel as it is not a spiritual being.
The Kiryat Sefer on the above cited Rambam writes:
כל מי שיש לו תפילין בראשו ובזרועו וציצית בבגדו ומזוזה בפתחו מוחזק הוא שלא יחטא שיש לו מזכירים רבים והם המלאכים המצילים אותו מלחטוא שנאמר חונה מלאך ה' סביב ליראיו ויחלצם:
All those who have tefillin on his head and forearm, and Tzitzis on his garment and a mezuzah on his door, strengthens him so that he does not sin, so that he has many reminders and they are the messengers that save him from sinning as it says "The angel of G-d camps around those who fear Him and protects them."
Therefore, I think it would perhaps be fair to view these articles as more of a G-d send (messengers and not angels), a heavenly reminder to act in the correct way befitting a Jew.
One has to be careful not to make the Mezuzah into an object that one views purely as a protective amulet and forget the source of our ultimate protection - G-d. Indeed the Rambam (Maimonides) writes earlier against this sort of belief.
He writes in Hilchos Mezuzah 5:4:
It is a common custom to write [God's name,] Shad-dai, on the outside of a mezuzah opposite the empty space left between the two passages. There is no difficulty in this, since [the addition is made] outside.
Those, however, who write the names of angels, other sacred names, verses, or forms, on the inside [of a mezuzah] are among those who do not have a portion in the world to come. Not only do these fools nullify the mitzvah, but furthermore, they make from a great mitzvah [which reflects] the unity of the name of the Holy One, blessed be He, the love of Him, and the service of Him, a talisman for their own benefit. They, in their foolish conception, think that this will help them regarding the vanities of the world.
So in conclusion, we view the mezuzah as a clear visual reminder to act in the correct manner, knowing that if we do, Hashem (and no-one else) will protect and help us.