Yes ads tend to be idealised and misleading, but that doesn't mean we shouldn't question a particular ideal, where it comes from and what it does to people. If you think "meh, well of course she looks like that, she's idealised" you're failing to see that the question is why THAT ideal. Why does the "ideal" for women have to be, generally, very thin, toned, white, often blonde and with no fat, but big tits? This is not only very unusual, and very hard for most women to ever become like, but may not even actually be healthy except for a few people. Many women held up as ideal/attractive - models and film stars etc. – are underweight and not well, as shovetheholly's post described.
If you think "oh well of course that's the ideal, of course anyone would like to look like that on the beach and of course a diet pill company will use that as their image" - you've internalised the idea that that's what a perfect and healthy woman should look like and what we all want to look like.
Many, many perfectly normal and healthy women loathe their own bodies. This is a big part of why – not this ad, on its own, but this phenomenon and the way so many of us internalise it and perpetuate it. It's really important to ask why, where it came from and what it says about how women are seen in general.
Feminists sometimes try to break this type of message down and see what it is really saying to women:
Women: be smaller, take up less space.
No matter how hard you try, you'll probably never be good enough.
But you must keep trying and divert your attention and funds to the effort to look thinner, more "attractive" (as defined by this image) and more acceptable to society, on the beach/wherever.
If you don't fit this image, you should feel bad about yourself. You're ugly and looking at you will offend people.
If you don't fit this image, people will be within their rights to sneer at and abuse you for your failure.
If you do, you'll be praised for meeting accepted standards of attractiveness and your actual achievements won't matter.
I'm not suggesting the ad company set out to say this, it's the patriarchal society that this is a product of that says this to women. The net result is that women suffer and are held back – THAT's why it's important. Not because I want to be Ms Outraged of Munsnet about one little old ad. Because the whole situation matters.