Why VB6.0 form displays as C# form?

If you have a window handle for the form (from the COM DLL) then you can disable visual styles on that form using the Win32 API:

SetWindowTheme( hwnd, "", "" );

I believe you’ll have to P/Invoke the API. Here’s the definition:

[DllImport("uxtheme.dll", ExactSpelling=true, CharSet=CharSet.Unicode)]
public static extern int SetWindowTheme(
   IntPtr hWnd,
   String pszSubAppName,
   String pszSubIdList);