She is probably just teething, and a bit off her food.
My youngest often refuses the high chair as well. In his case he wants to sit on a proper chair like everyone else. I'm pretty sure my other 2 did similar at this sort of age. My Mum says he will go in the high chair for her if he is hungry and the dinner is already on the chair waiting, otherwise nope!
He quite likes sitting at teeny table on a teeny chair we have, but mainly with us.
I honestly don't think respecting your LO wishes about where they sit makes them a 'spoiled brat'. It's just a toddler expressing their opinion. And high chairs are pretty unappealing things really
Oh, and try not to worry about the weight/ percentile thing too much unless they are losing weight. My 3 kids are all small - 2nd, 25th and 10th centiles, and honestly the more you worry and fuss about food the less they eat. Some kids are just more petite. My 2nd centile kid is eight now. He caused a lot of worry with reflux, failure to thrive, and being a super picky toddler, but eats well now, and is still very teeny, but he is a fab athlete. Plays county level tennis and gets on all the school teams etc. He's just a super small kid!