You must have gone to the Elves for advice, for the answer is both "no" and "yes".
No, they are not Maiar because they lack essential traits of the Maiar. In the Silmarillion, it says:
With the Valar came other spirits whose being also began before the World, of the same order as the Valar but of less degree. These are the Maiar, the people of the Valar, and their servants and helpers.
Now the Valar took to themselves shape and hue; and ... they took shape after that manner which they had beheld in the Vision of Iluvatar, save only in majesty and splendour. Moreover their shape comes of their knowledge of the visible World, rather than of the World itself; and they need it not, save only as we use raiment, and yet we may be naked and suffer no loss of our being.
Therefore with the Valar came other spirits whose being also began before the World, of the same order as the Valar but of less degree. These are the Maiar, the people of the Valar, and their servants and helpers.
This strongly suggests that the Maiar also could "walk unclad" and take to themselves "shape and hue". We also know that Sauron could do so and that he was a Maiar. Yet it seems pretty clear that neither Gandalf nor Sauruman could.
So they, therefore, were not Maiar at least while they were embodied in Middle-earth.
But also, "yes" they were still Maiar: We know so little of the nature of the Valar and Maiar that an equally consistant and reasonable answer is that they were Maiar who accepted, for a time, a diminution of their power, but were still Maiar. Is not a human who for a time wears a blindfold still human even though suffering a significant diminution of power?
Ultimately, the answer depends on details of the nature of the Maiar and the Istari that we simply don't know.