Why is a Trump attorney called a fixer, and a Clinton attorney is called a attorney?

Well, this is the case because one is comparing apples and oranges.

A fixer is a specific type of job (think Olivia Pope in the tv show Scandal) who is responsible for subtly negotiating problems.

A lawyer is a member of the bar of a State who represents a person on a legal matters.

So, while Michael Cohen is/was a barred member of the law community he was not acting as a lawyer for President Trump.