Is it bad to copy Apples ui designs?

Depends on what you 'mimic' and what you do with it.

There are certain elements that Apple expects the dev to honor, and it's good when 'kinda like' is done correctly. Where devs get in trouble is doing something different with things users have been trained to consistently expect to happen in ways that Apple intends.

See the HIGs, just keep in mind they represent significant info, so be prepared to spend time going thru them.

As for fonts, SF Font is common now...the system uses it, but devs are not allowed to use it for their own UI elements.

What 'stuff' did you have in mind?