Was Rambam a Karaite?

The ostensible answer to that question would be no.  He was the Rabbi and law maker par excellence.  Indeed, he writes against the Sadducees and Karaites, and also devotes many chapters to the strengthening of the oral law and rabbinic laws/additions.

However, there is a counter-current where he seems to be saying the opposite of all his rabbinic notions. In his legal corpus, the “Mishneh Torah”, a section known as “De’ot”, he writes  the following:

Ch. 3:

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A person might say, "Since envy, desire, [the pursuit] of honor, and the like, are a wrong path and drive a person from the world, I shall separate from them to a very great degree and move away from them to the opposite extreme." For example, he will not eat meat, nor drink wine, nor take a wife, nor live in a pleasant home, nor wear fine clothing, but, rather, [wear] sackcloth and coarse wool and the like - just as the pagan priests do.
This, too, is a bad path and it is forbidden to walk upon it. Whoever follows this path is called a sinner [as implied by Numbers 6:11's] statement concerning a nazarite: "and he [the priest] shall make an atonement for him, for his having sinned regarding [his] soul." Our sages declared: If the nazarite who abstained only from wine requires atonement, how much more so does one who abstains from everything.
Therefore, our Sages directed man to abstain only from those things which the Torah denies him and not to forbid himself permitted things by vows and oaths [of abstention]. Thus, our Sages stated: Are not those things which the Torah has prohibited sufficient for you that you must forbid additional things to yourself?
This general statement also refers to those who fast constantly. They are not following a good path, [for] our Sages have forbidden a man to mortify himself by fasting. Of all the above, and their like, Solomon directed and said: "Do not be overly righteous and do not be overly clever; why make yourself desolate?" (Ecclesiastes 7:16).

א

שמא יאמר אדם הואיל והקנאה והתאוה והכבוד וכיוצא בהם דרך רעה הן ומוציאין את האדם מן העולם אפרוש מהן ביותר ואתרחק לצד האחרון עד שלא יאכל בשר ולא ישתה יין ולא ישא אשה ולא ישב בדירה נאה ולא ילבש מלבוש נאה אלא השק והצמר הקשה וכיוצא בהן כגון כהני העובדי כוכבים גם זה דרך רעה היא ואסור לילך בה המהלך בדרך זו נקרא חוטא שהרי הוא אומר בנזיר וכפר עליו מאשר חטא על הנפש אמרו חכמים ומה אם נזיר שלא פירש אלא מן היין צריך כפרה המונע עצמו מכל דבר ודבר על אחת כמה וכמה לפיכך צוו חכמים שלא ימנע אדם עצמו אלא מדברים שמנעתו התורה בלבד ולא יהא אוסר עצמו בנדרים ובשבועות על דברים המותרים כך אמרו חכמים לא דייך מה שאסרה תורה אלא שאתה אוסר עליך דברים אחרים ובכלל הזה אלו שמתענין תמיד אינן בדרך טובה ואסרו חכמים שיהא אדם מסגף עצמו בתענית ועל כל הדברים האלו וכיוצא בהן צוה שלמה ואמר אל תהי צדיק הרבה ואל תתחכם יותר למה תשומם:

Interesting is the counter-statement of the Jerusalem Talmudic Sages, which opposes that of the Babylonian Talmud – “Are not those things which the Torah has prohibited sufficient for you that you must forbid additional things to yourself?” (Jerusalem Talmud, Nedarim 9:1).

The argument could be made that this is referring to personal vows, and that what the Rabbis forced is valid. However, what applies to individuals also applies to other individuals. The message of the Jerusalem Talmud is giving a rationale to the Torah prohibition of adding.  The violaters of this principle, say the reverse, to justify the adding of laws and restrictions.
Rambam wrote a compilation of halachic rabbinic Judaism from various sources, and there is no guarantee that his writing is internally consistent (after all the Talmud is not without contradiction).  It is sufficient, however to take this point of view into account, to show that even the Talmudic rabbis of the Yerushalmi  had severe reservations about the newish Testament of the oral law.