Honestly, it's really common for their appetites to drop around this age as their growth slows down a bit, so they appear "picky" as they just don't need as many calories and aren't as hungry and can hold out for the foods they most prefer.
Personally, I'd consider what you want dinner to look like long-term (balanced with what you consider age-appropriate) and establish that as the expectations and then not sweat it if she doesn't eat much. I have a 21 month old and a 4 year old and they both eat big breakfasts, medium lunches, and very little dinner--from reading posts around here, this seems quite common.