Some scholars, notably Dan Lusthaus, have argued that Yogācāra is not a form of Idealism. Lusthaus is one of the leading living authorities on Yogācāra and author of the authoritative analysis, Buddhist Phenomenology. The charge of Idealism is simply a mistaken reading of Yogācāra. For example in the introduction to his paper, What is and isn't Yogācāra he argues
Yogācāra focused on the processes involved in cognition in order to overcome the ignorance that prevents one from attaining liberation from the karmic rounds of birth and death. Yogācārins' sustained attention to issues such as cognition, consciousness, perception, and epistemology, coupled with claims such as "external objects do not exist," has led some to misinterpret Yogācāra as a form of metaphysical idealism. They did not focus on consciousness to assert it as ultimately real (Yogācāra claims consciousness is only conventionally real since it arises from moment to moment due to fluctuating causes and conditions), but rather because it is the cause of the karmic problem they are seeking to eliminate.
However, some scholars, for example Sean Butler, in his essay Idealism in Yogācāra Buddhism *, remain unconvinced by Lusthaus's argument. Butler cites Vasubandhu's Trimsika for example (and here he cite's Lusthaus's own translations):
The transformation of consciousness is imagination. What is imagined by
it does not exist. Therefore everything is representation-only.
For consciousness is the seed of everything. Transformation in such and such ways Proceeds through mutual influence, so that such and such imagination is born.
A note of caution is that Butler seems to be confused by Kant's Transcendental Idealism, which is not really a form of Idealism, in that it does not argue that the mind is all there is, but does argued that experience is all that can be known (Transcendental Idealism is not an ontological theory in the way that Idealism is). But he does go on to discuss more relevant definitions of Idealism. Butler weighs the arguments for and against labelling Yogācāra as Idealist and considers that he has decisively shown that it is.
Although Yogācāra is not my area of speciality, I am an advocate for Dr Sue Hamilton's hermeneutic strategy of treating all Buddhist doctrines as though they are addressing the topic of what we experience rather than what exists, which she outlines in Early Buddhism a New Approach. This outlook is in fact evident in Early Buddhist texts. As Bhikkhu Bodhi says:
“The world with which the Buddha’s teaching is principally concerned is ‘the world of experience,’ and even the objective world is of interest only to the extent that it serves as that necessary external condition for experience.” (Bodhi 2000, The Connected Discourses of the Buddha: 394, n.182)
If we read the early Buddhist texts as being concerned with experience, or in Buddhist terms with the arising and passing away of mental states, then a number of confusions are resolved. We can for example dispense with the confusing Two Truths doctrine that phenomena are both existent and non-existent. Early Buddhist texts take the view that when it comes to experience, ontological commitments like "existent" (atthitā) and non-existence (n'atthitā) simply do not and cannot apply. This need not imply a commitment to the proposition that only mental states exist (which is what Bishop Berkeley claims). But like Kant, in the Critique of Pure Reason, we might have to conclude that there is an epistemological limit on what we can know, since all of our knowledge comes via experience.
There is accumulating evidence that this early Buddhist outlook continues into the Prajñāpāramitā literature (See my long essay Form is (Not) Emptiness). So that it might persist into the Yogācāra ought not to surprise us. And for this reason I am predisposed to give Lusthaus the benefit of the doubt and accept his assertion that Yogācāra is not Idealist because it sees vijñāna as only conventionally real, in line with the Yogacāra interpretation of the Two Truths, which is distinctive to the Yogācāra. (Incidentally Lusthaus has written a good article on this subject: The Two Truths (Saṃvṛti-satya and Paramārtha-satya) in Early Yogācāra. Despite the philosophical problems entailed by the Two Truths, the fact that Yogācārins accept the consequences of their version of the doctrine makes it unlikely that they fell into the Idealist trap.
However one has to weigh the criticism of Butler and others. I think the jury is still out.