I think in spite of many other similar question like:
- What / how about this plain shirt?
- What do you think about this plain shirt?
and so on.
the sentence: "How do you like this plain shirt?" implies that the question maker has a semi-positive opinion about the shirt in his / her question, but they doubt about it and need to know about the second person' opinion.
Do you confirm it?
Best Answer
I have to disagree. Grammatically, the sentence is doing no more and no less than asking to what extent the other person likes the shirt.
Compare: How do you like your boss?
This does not imply any favor or disfavor on the part of the person asking.
That said, practically speaking, when you ask someone about a shirt, you are typically considering buying/wearing it, and thus typically have at least a semi-positive opinion of it. But that's true of all your example sentences.