We see many temples of Lord Vishnu, Rama, Krishna, Narasimha. Also temples for Varaha, Kurma, Hayagriva etc.. Are there any temples for Mohini avatar? Also, which avatars don't have temples and why?
Temples dedicated to Mohini Avatar are very rare. There are two temples where Mohini avatar is presiding deity.
Jaganmohini Keshava Swami temple located at Ryali, Konaseema, Andhra Pradesh.
Mahalasa Narayani temple. Mardol, Goa.
Mahalasa Narayani is identified with Mohini Avatar of Lord Vishnu. She is family deity of many families. Goddess Mhalsa is considered as Mohini as well as Parvati. Even though Mahalsa is identified with Mohini in the shrine, some traditions consider Mhalsa as incarnation of Goddess Parvati.
So, that makes Jaganmohini Keshava Swamy a unique temple for Mohini Avatar.
The idol in the temple is made of a single salagrama stone (Salagrama Ekashila). The deity looks Vishnu from front view and Jaganmohini with long hair when looked from the rear.
The temple history is associated with the Samudra Manthan episode from the Puranas. Following is the sthala Puranam (temple History from East Godavari District website):
According to the legend “Bhagavatam” while Devatas and Rakshas were quarrelling over sharing of holy Devine nectar “Sree Maha Vishnu” came to the rescue of Devatas in the guise of Mohini and convinced both the rivalry groups promising to distribute holy Devine nectar in equal share to Devatas and Rakshas. But in the interest of universal peace and welfare of sages, holy and Devine nectar was distributed among Devatas alone and the Mohini disappeared.
Lord Siva having seen the most fascinating beauty of Mohini allured her. He chased her for getting for a while the presence of his consort Parvathi Devi it is the general belief that the holy incident was the result of birth of “AYYAPPA SWAMY”. One flower from the plait of Mohini fell down and it was smelt by Lord Siva. Then he surprisingly found “Sree Maha Vishnu” in the form of Mohini and felt shy for his behavior. The place where the flower from the plait of Mohini fell is named as RYALI the Telugu meaning of “Fall”
Almost every incarnation have temples. Even Parashurama has a few temples dedicated and worshipped. Lord Brahma has only few temples and probably new temples are not constructed due to the curse of sage Bhrigu. According to Shiva Purana, it is Lord Shiva who banished Brahma from temple worship and only gave the presidency over all the Yagnas.
Wikipedia article on Mhalsa
Temple History of SRI JAGANMOHINI KESAVA & GOPALA SWAMY TEMPLE, RYALI from East Godavari district official website.
Ryali, Lord Jaganmohini Keshava Swamy Temple from inkakinada.com
This is just an addition to Sarvabhouma's answer.
Padma Purana, Uttarakhanda, In Chapter 241 Lord Siva relates the story of Parasurama to mother Parvati.
79-81. O goddess, I have told you this account of the incarnation of lord (Visnu), holder of the Sarnga bow, who had entered (Parasu)Rama with his power. Due to the entrance of the power this (form) of the noble one cannot be worshiped. The chief, noble, brahmanas, the devotees of the lord, should worship the incarnations of (Dasarathi) Rama and Krsna, perfect with virtues, worshiped by sages, and giving salvation to human beings.
From this what we can infer is that sakty-avesa avatars need not be worshiped(although there may be some temples). Avatars like Kurma, Rama, Varaha, Hayagriva, etc.. should be worshiped.