Would it be ethical to hire a proofreader for theses and academic articles?
Short answer

Yes, it is ethical

Long answer

The three options are:


Proof-reading it yourself
Asking a co-author (for academic article) or a 3rd person (for theses) to proof-read it
Pay for a commercial proofreading service.


As I see it:


Option (1) is inefficient. You've written the text, read it many times over, and your brain will simply fool you in not seeing the typo's.
Option (2) is done quite often in practice. In my opinion, it's most suitable for journal articles; some articles have quite a lot of co-authors, and one could give a section to each co-author and distribute the work. That's what I usually do (my thesis will be quite small, because it's just a summary for a sandwich thesis). 
As for option (3), I don't see any ethical problems with paying for a proofreading service. I know a number of people (they are not native speakers) who do this systematically for theses and academic articles. They buy commercial services who check and correct the quality of their English writing. This improves the quality of the text and therefore reduces the chance to annoy the reviewers.