Was Marie Antoinette as dumb and selfish as history paints her?

Not at all!

The image of her as oblivious to the suffering around her is incorrect. She was charitable, and sympathetic to the plight of the poor, and she tried to raise her children to be, too. The memoirs of Madame Campan have more details. Link to part 1, and part 2.

She wasn't very interested in subjects traditionally seen as intellectual, and her early education was lacking. According to Antonia Frasers biography, she didn't like to be around people who were overtly intelligent. However, that doesn't mean that she was stupid. She was deeply involved with the politics of the court, and did what she could to influence events in Europe in line with her interests, corresponding secretly with other European monarchs and using her position to gain access to military secrets that would help them in the war against France. During her trial, she showed both her wits and her strength of character, cleverly answering the questions asked without either implicating herself or betraying her beliefs.

Most people here probably know that the "let them eat cake" quote isn't accurate, and instead attribute it to revolutionary slander. However, I've never seen any mention of this quote in any source from the time, or any mention of it being connected to her during her lifetime. Republicans were more likely to attribute her behavior to counterrevolutionary plotting than to stupidity. The act of accusation against her (link in French) says

[...] the public prosecutor has raised this accusation against Marie Antoinette, in her interrogation naming herself de Lorraine d'Autriche, widow of Louis Capet, for having maliciously, and by design,

in concert with the brothers of Louis Capet and the infamous minister Calonne, squandered the finances of France in a horrifying manner, and for having sent incalculable sums to the emperor (i.e. her brother, either Joseph II or Leopold II), and for having thus dried out the national treasury.

Earlier on in the same document we're told that part of this terrifying spending was to satisfy her "disorderly pleasures", but taken all in all I think what Fouquier-Tinville is trying to show here has more to do with plots against the state, and less to do with just really wanting more pretty dresses. Especially since her personal expenditures are always mentioned in connection with her sending money to France's enemies. Nowadays, most people don't know about her actual counterrevolutionary plots, which makes the Stupid Spoiled Princess trope an easier explanation for her spending.

In reality, her spending wasn't very noteworthy for a queen at the time, and considering that the king never had a mistress to spend money on, she could have afforded even more expensive habits. It certainly didn't put an actual dent in the state finances. Only 6% of the budget was spent by the court, while over 50% went to managing the debt. (Georges Lefebvre, 1789) Which is a lot of money! But most of it wasn't Marie Antoinette's choice. The court spent a lot to keep the nobles happy with their post-Louis XIV lack of political power with pensions and well-payed positions in the king's household, and the king couldn't have cut down on that without getting into trouble. The problem was also that the extravagance of the court served a political purpose, to glorify the king by displaying the wealth of the nobility and royalty. When public opinion turned against both, people were angered by their waste, instead of dazzled by diamonds and paniers.

Thank you so much for the gold!