This is a great question! It is not historically accurate, but it's based on an actual historical thing to some extent.

At the very beginning of the eighteenth century, there was a huge structural difference between English women's ordinary dress and women's court dress, a sort of uniform required for women at formal court events, like presentations. In everday life, they wore stays (corsets) and mantuas (fairly loose gowns that opened in the front), but for court events there was an older style - the norm for all dress construction until roughly the 1680s - with a separate boned bodice worn with a matching petticoat. Soon, this older style was worn only by royal women in cloth of silver for their wedding dresses, while court dress was a more formal version of the mantua, made with a long train that could be, and in fact was intended to be, worn folded up at the waist. In France and a number of other European countries, the boned bodice continued to be part of court dress even as it became more and more archaic.

In the 1720s, the hoop was introduced into English women's fashion. With a hooped petticoat, a woman could create a skirt in a shape completely impossible to achieve with layers of ordinary fabric underskirts - and while hoops were initially rounded, they came to take on a fairly flat, rectangular silhouette that is now the stereotype for the entire century. By the 1770s, the fashionable profile was much more moderate and rounded in comparison.

By the early 1780s, hoops like this were only worn in highly specific situations where they were required, which included court events as well as some balls and assemblies - women were instead typically filling out their skirts with padded supports worn around the hips. And soon they were not being worn to any events except those at court, because the waistline was rising and it simply looked silly. The overall aesthetic preference was for a "natural" look that somewhat followed the actual lines of the body, and hooped petticoats were the antithesis of that.

The thing about court dress is that it was under the mandate of the king. It had been set sometime previously as requiring hoops, ostrich feathers, and a skirt open over a petticoat, and so even as other aspects of the outfit changed with the fashions (colors, materials, the raised waistline, etc.) these were retained. That is, while it's very popular for Regency costume enthusiasts to say that hoops remained part of court dress because Queen Charlotte insisted on them, it was ultimately down to George III to put out an edict on the subject - and while she may have played a part in their retention, she was also credited in the period as having encouraged the end of the fashionable hoop as worn to non-court events. During the Regency proper, it was impossible for anyone to change court dress to get rid of the hoop, as the king was incapacitated and not capable of issuing edicts of this nature. The Prince Regent didn't wait until his mother's death in 1818 to bring English court dress in line with fashion and other European courts', as is often said - he did it as soon as he became king in 1820.

The Bridgerton costume designer believed, due to this frequently repeated history fanon, that Queen Charlotte loved the fashions of the 1770s and copied them in their entirety, and so represented the queen and her ladies as anachronistically dressed. At the same time, the other women attending the court function shown in the show are also anachronistically dressed when they're being presented as they are not wearing hoops!