Should "my comment" and "my thought" use singular form or plural form while commenting about a single item?

According to the usual rule a plural is used for the verb if the subject is plural; this is so in formal writing (basic rule); however, in the spoken language it is different: particular cases.

What is also needed in your formulation is italics for the title (ref.).

• Here is/are my comment/s on the book To Kill a Mockingbird.
• Here is/are my thought/s on the book To Kill a Mockingbird.

In the first sentence, if a simple comment is what has been asked then a singular should be used. If a lengthier piece of writing has been required of you, a plural is better.

In the second sentence, a singular would be something exceptional; given the rare possibility that a teacher asked you to mention in a short exercise a thought about a book, then the singular is proper. If the exercise has to do with developing on a single thought of yours, then this developing will consist of a succession of thoughts and in this case neither the singular nor the plural would be a wise choice, a change of term being the best you might do (Here are my idea on… (for instance)), but then the matter is not a whole book; so you are not concerned by that. If what has been asked is your thoughts, then, evidently you should use a plural (Here are my thoughts…).

The choice depends on the complexity of what you write and on the length you intend to give to your paragraph; you do not have to treat only one idea or thought in a given paragraph. You more or less must treat a short subject, involving possibly several connected ideas or thoughts.