Do they mean "These trousers are plain"?

Yes, something like that, "plain", "modest" , "nothing special".

I think its really up to how people personally think of 'simple' and 'modest'. Personally I see it as a positive great thing. Plain to me sounds negative tho. We both just interpret the word differently and its down to preference/personality for the context. A little known fact too that political allignments can be detected in how people structure sentences.