URL with letters in high-box

No. There is no problem.

But you should think about what will happen if a user tries to access http://exemplo.com.br/minha-pagina.php (all in lowercase)? Page not found or will open the same page?

According to the W3C (about HTML and URLs):

URLs are usually CASE-SENSITIVE, but there may be URLs (or parts of them) that are not case-SENSITIVE. users should always consider urls to be CASE-SENSITIVE.

However, we know that it is more difficult for the user to memorize which letters are uppercase or lowercase... it is easier to type everything in one form (upper or lower case).

So, thinking about what the user can type, it would be interesting to let the two pages be accessible, i.e. not CASE-SENSITIVE.

To have no problem with SEO (search optimization), define what will be the default format of the URL, and if the requested URL by user be different redirect (with HTTP Status: 301 Moved Permanently ) to the URL in the default format.

UPDATE: example of redirection in PHP with HTTP Status: 301 Moved Permanently.

$url_padrao = 'http://exemplo.com.br/Minha-Pagina.php';
Header( "HTTP/1.1 301 Moved Permanently" );
Header( "Location: $url_padrao" );