Mohini is not included in the standard Dashavatara (the ten principal incarnations of Bhagwan Vishnu) because she is the illusory form (maya-roop). Wherever Goddess Mohini is mentioned, the word Maya is always mentioned there and Maya is actually a key aspect of Shakti and hence Mohini’s emergence is explained broadly in Shaktism which is that branch of Sanatan Dharma which revolves around the female forms of the Divine.
Unlike other Vishnu avatars, Bhagwan Vishnu takes Mohini avatar any time. Also, since it is a female form it has a link with Devi. In other avatars, Narayan directly appears as a male being. So they are the main 10 avatars of Vishnu (Dashavataras). However, Mohini avatar is mentioned in the 24 avatars of Bhagwan Vishnu which is a broader list of incarnations.
It is interesting that Mohini is very often associated with Shakti because every female has been considered as the manifestation of Shakti and more importantly Shakti is Mahamaya, which strengthens Mohini’s connection with Devi further as Mohini is indeed describe as the manifestation of Maya. As per Brahmanda Purana, it is said Vishnu acquired the Mohini form from Goddess Maheshwari. So it actually Mahadevi manifesting through Bhagwan Vishnu as Mohini is completely female not that Shri Hari does some makeup to look feminine, that avatar is properly female and whatever is female, it comes under the domain of Devi. Further it establishes the oneness of Vishnu and Uma in a sense that the Ultimate Power which is Vishnu in male form, the female version of the same Ultimate Power is Uma as per multiple Puranic texts as well Upanishads that revolve around Advaita philosophy. That means all the deities we worship are actually the same Supreme Reality appearing in different forms. So it's like A=B, A=C so B=C and hence A=B=C.
Hayagriva avatar of Shri Hari Vishnu explains about Mahadevi Lalita Tripurasundari. From there comes, Lalitopakhyana, Lalita Sahasranama etc.
In Lalita Saharasanama, Mohini directly appears as the 562nd name of Lalita Devi which confirms that Mohini does have a connection with Lalita, the third among the ten Mahavidyas.
The way I see it is that other forms of Shakti represent the motherly nature of the divine (Parvati, Durga, Kali, Lakshmi, Saraswati etc.) or the faithful wife or the daughter. All of those forms are gentle and benevolent. Mohini on the other hand is a different version of Shakti, bold and the enchanting energy. Hence, the Mother Goddesses do not take up that form to deal with the Asuras but rather destroy them directly in battlefield. In Shaktism, all the Goddesses are considered as the form of Lalita, so Mother Goddesses are also her form and the enchanting Mohini is also her form, the only difference is that Mother Goddesses don’t transform into the enchanting Mohini for dealing with Asuras, so that form against Asuras is taken by Bhagwan Vishnu.
Thus, Mohini connects really well with God Vishnu as he is the one who tricks the Asuras.