Noooo, opposite way round! IME, it's the slim, glam-ish types who look effortlessly cool in a simple t-shirt, flat shoes and jeans.
It's the more "homely" types, to use your choice of words, who aren't as confident or as used to going to gigs, so try and dress up with boots and a big necklace, but if I'm honest, I think that looks a bit too much "mum-at-a-gig."
Depends who you're going to see though. If it's the likes of Coldplay at an arena, then everyone will be in ankle boots, biker jackets, chains, drapey vests etc. Clearly dressing up for a one-off gig. There's usually lots of women looking pained in heels by the end of the night.
But if a local band at an Academy/smaller venue, dressing like that would make you look like you were there to pick your teenagers up. Hence the suggestion of keeping it simple and casual - the aim is to look like you've made minimal effort.
Sorry if that sounds harsh, just that one type of gig is not the same as another!