Is paying someone to fix LaTeX issues in one's thesis ethical?

I do not see any unethical point in this. You were supposed to make an original contribution to your thesis in terms of your research. With respect to writing, you can take help from your friends, supervisor, or anyone else (free or paid).

In our university in New Zealand, there was a dept. which used to charge a small fee to fix language issues for students with English as a second language. I suppose this should also work for the images/pictures thing as you are not an artist but an expert in your particular field.

Cutting it short, I do not see anything unethical in it.

This is quite common. Some Universities in the UK are hiring private companies to improve the presentation of grant proposals, which includes typesetting, proofreading, professional preparation of graphics, and in many cases editing the text. Academics are encouraged to also use these companies to help improve their papers before they are submitted to a journal / conference (the Universities won't pay for this but academic can pay from their grants).

This practice does not quite fit in a popular vision of academy as a world where personal skills and commitment are more important than social background, money, old boys network, etc. It appears that those with money, fame and powerful friends have better chance to be supported, win the competition and get even more money, fame and powerful friends. Personally, I find it frustrating, but on a global scale this is acceptable and is not considered unethical or morally wrong.

I also see no big problem here. Generally, there is nothing wrong with accepting help for any part of your thesis, I would even encourage everybody to discuss and refine their work with others from the field or the group. A paid service is nothing I would usually recommend, but it is also possible.

You however must make the support (paid or not) you got transparent, especially if you have to include a statement which says that you disclosed all help you got. Then it is for the graders to decide if your thesis justifies the top grade. In your specific case you could have mentioned the services you used in the acknowledgements.