Showing awareness of critical response to texts is an important marking criteria for English exams - showing you have considered a range of critical opinions is a brilliant way to push your essays into the next level. Whilst it is fine to simply reference the opinion of a critic through quotation, what shows a higher level of sophistication and engagement is to query the critic's point - or at least to show an awareness of a range of opinions. Therefore it can be helpful to consider if the critical opinion you are referencing belongs to a particular tradition of literary thought (ie. is it a Marxist critic?) or a biased perspective? Alternatively you could refer to a critic's comments on one work and show how this is also relevant to another text by the same author - or a contemporary work.