Zaydis also do takfir against Muawiyah so here is a reply to your question.
> Bismillah,
>
> We class anyone who says" La Ilaha illa Allah, Muhammad Rasul'Allah" as
>a Muslim, regardless of the sect he/she belongs to under the umbrella of
>Islam.
> While we disagree with the Zaydi Shia sect about Muawiya, but that
>disagreement does not invalidate their Islam.
> In fact, the Takfir of Muawiyah was done by some Sunni Ulama such as the
>late Hafeth Sayyed Ahmad bin Assiddiqq, and many other Shuyukh in the past.
> We view that wrong, plain and simple.
>
> Yet, all the Shia sects, 12'ers, Zaydi's and others, including the
>Khawarej, most of the Mutazilah view Muawiyah as non-Muslim, or at a red
>light.
>
> The reason for Sidi Hafeth Ahmad bin Assiddiqq's view is a few Ahadith
>that are authentic in his opinion. Such Ahadith have been narrated and they
>explicitly and clearly declare Muawiyah as non-Muslim. Among the strongest
>narration is one by Imam Balathiri in his Tarikh, with a sanad all of which
>the narrators of Imam Muslim, hence Hafeth Ahmad declared him Kafer.
> This hadith needs a Hafith to authenticate otherwise it is discarded.
>Obviously Hafeth Ahmad authenticated it, as all its narrators as the same
>narrators of Imam Muslim. Yet other scholars cast doubt on the whole
>narration, not the isnad.
>
> Another reason Hafeth Ahmad and Zaydi's declared Muawiyah as Kafer is:
>the condirmed act of Muawiyah of cursing and ordering the cursing of Imam
>Ali. Such Ahadith are narrated in authetic isnaad in Sahih Muslim, and
>Sunun Ibn Majah, and almost every single Islamic history reference. If you
>add that fact to the hadith Al Hakem narrated which says: Who curses Ali
>curses me, and who curses me curses Allah", you would see clearly who
>Hafeth Ahmad and the Zaydi's and others went to such extent. There are many
>other Ahadith to that effect also and countering them as well.
>
> While disagreeing with such result, we see it as wrong, as it is based
>on weak evidence, and the best judgment in this case should be more careful
>and giving the benefit of the doubt, and sticking with most of the
>narrations that are authentic that did not mention such thing, on the
>contrary.
>
> Bottom line:
> Since those groups and scholars depend on an explicit prophetic
>narration in their judgement, we cannot declare them anything because of
>this opinion specifically. Though we disagree and believe that such a
>narration cannot possibly be true, besides, confirming such thing or
>denying it, will not affect our own Iman, nor will it increase it
>personally, and Allah will not ask us about the Iman and deeds of Muawiayh.
>
> The best and safest is to stick to Ahlus Sunnah's stands, and Allah
>knows best.
> Our stand is to consider Muawiyah a Sahabi without overglorification,
>and avoid entirely the events or wrongs he did. Also we view that cussing
>him or calling him names in not permissible and sinful.
>
> Shaykh Ahmad Husain Kamel