I think charities spending a lot of money on advertising can occasionally backfire from a PR point of view, but it’s fair to say that you have to look at whether the advertising is actually effective or not.
If they spend £1,000, but receive extra donations of £2,000 as a result, then presumably that’s a good thing. It also depends if the advertising’s any good or not: if so, it increases revenue to allow them to continue the good work that they do.
My assumption would be that people object less to money being spent on marketing and advertising per se, but rather on excessive bureaucracy and inflated salaries within charities. There are then the questions of how much should directors of charities should be paid and whether there should be limits, but thankfully you did not ask me that!