感想文書くとき丁寧体で書かなければなりませんと聞いたことがありますが、
日本製品に表れる思いやりの気持ち
について書くとき丁寧体をつかなければなりませんか。教えていただけないでしょうか。

See a translation

There is no such rule. However, when you write something, the writing style would better be consistent, because that makes the readers comfortable. Typically in Japanese schools, teachers tell the students to use sentences ending with です and ます or those ending with だ and である. If you mix them, the writing sounds inconsistent.

Having said that, it is also true that there are Japanese novels and articles that deliberately use inconsistent writing styles and enjoy the sense of mixture.

Therefore, personally I would suggest you write just as you like without worrying too much about it. However, in classroom settings, I understand that teachers want Japanese learners to use what they call 丁寧体 because that is the orthodox style of writing.

All things considered, my opinion is that you would better follow the guidance from your teacher if it is something you need to submit to the teacher. If you don't need to, then you can try using different styles of writing and hone in your writing skills.