In most cases, I would say:
Yes, when writing a Service Design Principle, reveal your opinions.
Sharing also your personal opinions has a few benefits:
It helps you better understand what you think
If you share your principles, people can react to them, and through the conversation, maybe your opinions will change and evolve
Opinions make the reading of principles a more human experience. It feels more like a conversation than an academic paper.
There are two cases where you might have to counter balance your opinions more strongly when writing a Service Design Principles:
If you're taking an academic approach
If you're writing these principles within a larger organization
Inspired by a community question
Thanks to Ramya Mahendran, who inspired this text by sharing a bit of her own writing journey.