Our DD, being a child from our poor family, had a simple shirt with a simple skirt. Both of them prabably from yard sale for less than $2.
On the ceremony, a total of more than 30 awards were given out, some for individual and some for group. As one of the graduting class of about 200 students, DD won about 15 awards out of that 30. A one point of time, she got three awards in a row.
When they stood side by side, I don't felt DD was having any less 自信 than the next girl with full make up and expensive jewelry. You can tell she is full of 自信 from the way she smiles.
Nice clothes are definitely a plus. However, I don't think it generates 自信. A feeling of superior, may be.