Truth to tell, I've always had a collection of ideas on Mrs Allan which are rather vague, so I hope this isn't too rambly.

Don't get me too wrong but I think materialism literally defines Mrs Allen. Different kinds of materialism appears to be a theme of NA.
How Mrs Allan obseseses over shopping, muslims, and fusses over her gowns and getting ready for the assembly ! ;)
When Mrs Thorpe brags about her wonderful children, the childless Mrs Allan takes comfort Mrs Thorpe's lace isn't so handsome as her own.
I too, think she'd rather be noticed for her attire than herself. She'd rather new gowns wore her than she wore the gowns !
She can be delightfully vague.
Yet Mrs Allan can be decided on dress. At the Upper Rooms, she shows some decided views about gowns- well, I guess it was part of a chaperone's duty to teach her young charge about shopping and appropriate dress !
Actually, I used to wonder about the Morlands' decision to let Catherine go to Bath under chaperonage of Mrs Allan.
Yet the narator tells us of the limits of Fullerton for a young lady, Mrs Morland is still coping with several younger children at home. The opportunity of Bath was not to be given up lightly, Mr Allen was a sensible man.
I believe, as a lady Mrs Allen is a kindly old dear whose genuinely fond of Catherine. Yet if she'd known how to do her job as a chaperone in public, maybe she could've asked Mr Allen to ask the MC to introduce Miss Morland to some other young ladies whom they may've seen again at the Assembly Rooms and at functions. Mrs Allan may have built up a large acquitance she wanted!
I know Mrs Allen had no acquitance and was wholly inexperienced at being a chaperone. Her faults as a person are so harmless. Pity the Thorpes showed up ! ;) Despite the air of a gentlewoman, some of her lines are quite hilarious. Really, I can't help liking Mrs Allan.