Reform Jews believe in the messianic age but not specifically in a personal miraculous messiah. According to Rambam's 13 principles of faith, is this heresy? Are Reform Jews heretics?
The question of "reform jews" being heretic would be no different from a specific Jew being heretic (choss vishawloym), there are rules for determining who is considered such, if those rules and conditions are not fulfilled, regardless of whether the Jew identifies as "reform" (whatever exactly that would imply) or not doesn't generally make a difference
In the Rambam, beginning of hilchos mamrim, the basic criteria for who is considered a heretic are given, and he explicitly excludes those who were raised and brought up in a heretical society
A person who does not acknowledge validity of the Oral Law is not the rebellious elder mentioned in the Torah. Instead, he is one of the heretics and he should be put to death by any person.
א מִי שֶׁאֵינוֹ מוֹדֶה בַּתּוֹרָה שֶׁבְּעַל פֶּה אֵינוֹ זָקֵן מַמְרֵא הָאָמוּר בַּתּוֹרָה. אֶלָּא הֲרֵי זֶה בִּכְלַל הָאֶפִּיקוֹרוֹסִין [וּמִיתָתוֹ בְּכָל אָדָם]:
2 Since it has become known that such a person denies the Oral Law, he may be pushed into a pit and may not be helped out. He is like all the rest of the heretics who say that the Torah is not Divine in origin, those who inform on their fellow Jews, and the apostates. All of these are not considered as members of the Jewish people. There is no need for witnesses, a warning, or judges for them to be executed. Instead, whoever kills them performs a great mitzvah and removes an obstacle from people at large.
ב מֵאַחַר שֶׁנִּתְפַּרְסֵם שֶׁהוּא כּוֹפֵר בַּתּוֹרָה שֶׁבְּעַל פֶּה [מוֹרִידִין אוֹתוֹ] וְלֹא מַעֲלִין וַהֲרֵי הוּא כִּשְׁאָר כָּל הָאֶפִּיקוֹרוֹסִין וְהָאוֹמְרִין אֵין תּוֹרָה מִן הַשָּׁמַיִם וְהַמּוֹסְרִין וְהַמּוּמָרִין. שֶׁכָּל אֵלּוּ אֵינָם בִּכְלַל יִשְׂרָאֵל וְאֵין צָרִיךְ לֹא לְעֵדִים וְלֹא הַתְרָאָה וְלֹא דַּיָּנִים [אֶלָּא כָּל הַהוֹרֵג אֶחָד מֵהֶן עָשָׂה מִצְוָה גְּדוֹלָה וְהֵסִיר הַמִּכְשׁוֹל]:
3 To whom does the above apply? To a person who denied the Oral Law consciously, according to his perception of things. He follows after his frivolous thoughts and his capricious heart and denies the Oral Law first, as did Tzadok and Beitus and those who erred in following them.
The children of these errant people and their grandchildren whose parents led them away and they were born among these Karaities and raised according to their conception, they are considered as a children captured and raised by them. Such a child may not be eager to follow the path of mitzvot, for it is as if he was compelled not to. Even if later, he hears that he is Jewish and saw Jews and their faith, he is still considered as one who was compelled against observance, for he was raised according to their mistaken path. This applies to those who we mentioned who follow the erroneous Karaite path of their ancestors. Therefore it is appropriate to motivate them to repent and draw them to the power of the Torah with words of peace.
ג בַּמֶּה דְּבָרִים אֲמוּרִים בְּאִישׁ שֶׁכָּפַר בַּתּוֹרָה שֶׁבְּעַל פֶּה בְּמַחֲשַׁבְתּוֹ וּבִדְבָרִים שֶׁנִּרְאוּ לוֹ. וְהָלַךְ אַחַר דַּעְתּוֹ הַקַּלָּה וְאַחַר שְׁרִירוּת לִבּוֹ וְכוֹפֵר בַּתּוֹרָה שֶׁבְּעַל פֶּה תְּחִלָּה כְּצָדוֹק וּבַיְתּוֹס וְכֵן כָּל הַתּוֹעִים אַחֲרָיו. אֲבָל בְּנֵי הַתּוֹעִים הָאֵלֶּה וּבְנֵי בְּנֵיהֶם שֶׁהִדִּיחוּ אוֹתָם אֲבוֹתָם וְנוֹלְדוּ בֵּין הַקָּרָאִים וְגִדְּלוּ אוֹתָם עַל דַּעְתָּם. הֲרֵי הוּא כְּתִינוֹק שֶׁנִּשְׁבָּה בֵּינֵיהֶם וְגִדְּלוּהוּ וְאֵינוֹ זָרִיז לֶאֱחֹז בְּדַרְכֵי הַמִּצְוֹת שֶׁהֲרֵי הוּא כְּאָנוּס וְאַף עַל פִּי שֶׁשָּׁמַע אַחַר כָּךְ [שֶׁהוּא יְהוּדִי וְרָאָה הַיְהוּדִים וְדָתָם הֲרֵי הוּא כְּאָנוּס שֶׁהֲרֵי גִּדְּלוּהוּ עַל טָעוּתָם] כָּךְ אֵלּוּ שֶׁאָמַרְנוּ הָאוֹחֲזִים בְּדַרְכֵי אֲבוֹתָם הַקָּרָאִים שֶׁטָּעוּ. לְפִיכָךְ רָאוּי לְהַחְזִירָן בִּתְשׁוּבָה וּלְמָשְׁכָם בְּדִבְרֵי שָׁלוֹם עַד שֶׁיַּחְזְרוּ לְאֵיתָן הַתּוֹרָה:
So the answer to the question is: it depends on each specific case
If one was brought up by aliens (idolatry) or in an idolatrous society and doesn't know, truly, what the Torah really means, even if s/he sees other people keeping it, then that person is not considered a heretic at all according to the above, even if they identify as a "reform" Jew
The case might be different for someone who definitely knows about the truth of the Torah and consciously denies Torah shibawl peh etc. like yushkuh or others from the earlier generations
Blessings and success