What I find of interest is Louisa is the same age as Anne when she became engaged to Wentworth; yet Louisa does appear childlike compared to Anne; running, skipping and wanting to jump off stiles and have CW catch her. She bears no comparison to Sophie Croft, I agree.
Yet, with Louisa's genteel, comfortable life at Uppercross she remains in a world as close as Anne's. Perhaps the worst about Louisa is she is very inexperienced and never had to cope with hardship.
I think Louisa's vivacity is a foil to the meditative Anne. Her pleasant, lively manner probably distacts Capt. Wentworth yet she is entirely unexceptional.