Indeed, the Preface can be considered a type of manifesto of Romanticism. It serves a declaration of intent as to what Wordsworth and Coleridge see as the direction of Romanticism as a movement. The idea of being able to construct language and meaning in a different way, one that features a "certain colouring of imagination," helps to reconfigure how these thinkers were going to change the role of art and the artist in its creation. The Preface goes a long way in being able to provide a sense of structure to Romanticism. The entire tone of the Preface is declaratory. There is little in way of complexity offered. It is a statement, a bold and constructive element that makes no hesitation about its intent to change people's minds about how they view art and how individuals view themselves. In this light, one can see this as a manifesto of Romanticism.
I think in your question, there is a problem with the expression 'romantic criticism'. There is a difference between the Romantic poetics and the Romantic criticism. The former is a theorization of the literary movement or school from within i. e. by its practioners. That is the idea of a manifesto. The 'criticism' may refer to seconday texts of critical reception of that school over a period of many years. The Preface to Lyrical Ballads was written by William Wordsworth himself and it is a movemental manifesto of Romantic thought, if I may put it like that. If it is so, these are some of the basic directions in it---
1. Poetry as a spontaneous expression.
2. Poetry as a major treatment of powerful feelings and thus a revival human emotions in it.
3. The language of poetry as a common day to day language, an effort to democratize poetry.
4. The workings of imagination in poetry.
5. The role of nature and God in harmony along with humankind---the subject of poetry.