No offence to Joan, because I'm sure she really believes that, but I think that comment is a bit offensive to the Queen Mum's memory.
The Queen Mum lived through experiences that Camilla never will.
She had to step up and become queen when her brother-in-law abdicated. I don't think she ever thought that was going to happen. There's no way she was prepared to be Queen of England. It changed her life completely, but she did it with grace and without complaint.
She remained in London when it was bombed repeatedly in the blitz during WW2. Buckingham Palace was even bombed and she didn't flee. She put on her nicest dress and hat and bravely walked through the rubble (there are many iconic pictures of this).
She had to deal with the crisis of her second daughter wanting to marry a divorced man (scandalous in those days!). She had to steer her back on the 'right' path. She also had to see her daughter pass away before her many years later.
She continued to do public duties well into her 90s! And always with a smile on her face. Retirement was not a word in her vocabulary.
I don't think Camilla will be remembered the same way. Partially, it's just due to time. Camilla isn't going to be a royal for 80 years like like the Queen Mum was. She won't have as much time to 'win over' the people with all her deeds. But Camilla hasn't been challenged with anything of note and just releases mealy-mouthed statements like "I'm just here to support my husband." She just comes off as very plain and boring, just sort of standing two steps behing Charles, following whatever his lead is. She may not of chosen to become royal, obviously it came with the marriage, but if she wants to be remembered in the same breath as the Queen Mum, she needs to seriously pick it up and be perceived as someone to be admired, who really brought a nation together in very trying times or else Joan's comments are going to sound pretty ridiculous 20 years from now.