Je to... is more appropriate when we already know about the existence of something and we have just identified it is a mouse
To je... is quite universal, but when continuing a conversation about something it will generally be better or may even be necessary to use "Je to... ". Do you know František? He is a good boy. Znáš Františka? Je to dobrý hoch.
It is a good cake! Both To je dobrý dort! and Je to dobrý dort! are fine as a spontaneous sentence. But when directly asked what you think about it you answer "Je to dobrý dort.".