Yes, a professional proofreader or editor can proofread your thesis. However, depending on your university, there may be limits to what a proofreader can do. I will explain it all in this article.
A thesis has to be the author's work. Others must not alter or comment on it, as it means that it is no longer the work of the student alone. Certain universities allow a small amount of "comments" regarding syntax, but it has to stay to a minimum. Other universities only allow a proofreader to check for spelling, grammar, and punctuation mistakes and nothing else. So, before you ask a proofreader for a quote, do your homework and tell the proofreader or editor what your university's guidelines are regarding professional editing.
I often see requests from students that state that they want recommendations for improvements. However, making recommendations for improvements is prohibited.
Does a proofreader need to understand my subject?
No, they do not need to understand what you are trying to prove with your thesis. A proofreader's job is to check for spelling, grammar, and punctuation mistakes and to ensure the sentence makes sense (to a certain degree). Your proofreader is not allowed to fact-check or edit your text as it will change its meaning, nor are they allowed to leave notes that may change the meaning, even if they notice something is out of place. This is your, the student/author's, responsibility.
Things to keep in mind.
You will need time to go through your proofreader's notes. Remember, it can take you several hours to days to go through all the edits, so it is important to allow yourself enough time to go through everything before you have to submit your thesis. Give your proofreader a deadline of at least a week before your submission date if you can.
You, as the author, are to make sense of your thesis. Your proofreader can remove those spelling, grammar, and punctuation mistakes and leave some notes on sentences that don't flow well, but they cannot significantly change your text or fact-check.
Check with your supervisor if you are allowed to hire a professional proofreader. If you can get authorisation from your supervisor and share this with your proofreader, you're two steps ahead, and your proofreader can get to work ASAP.
Good luck with your thesis or dissertation!