Last I heard the ads were introduced to be less dependent on Google money - they actually cover the costs/salaries of the internal MDN team and thus secure the existence of MDN within the organisation.
Also apparently they are non-tracking ads, and so provide only a small fraction of income that tracking ads would bring, but that would go against the ideals of Mozilla.
So I'm seeing the ads as a net positive. (And am surprised that the people visiting MDN don't use an ad blocker anyhow).
I support MDN by disabling UBO. This is not (much) of a burden: there is no animation, and the advertised products are always technical and possibly of interest. Only complaint is the occasional large light areas, which do not play well with dark mode.