Can preface to lyrical ballads be considered as a manifesto of romantic criticism?

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.