For to us a child is born, to us a son is given; and the government shall be upon[a] his shoulder, and his name shall be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace.
The phrase ‘Mighty God’ is Hebrew el gibbor. Gibbor means ‘mighty in battle’. El, though usually translated ‘God’, can also refer to powerful human figures such as ‘rulers’ and ‘warriors’. In this context el gibbor may be better translated ‘mighty warrior’ (cf. ‘in battle God-like’ [NEB]).
It is also worth remembering that this prophecy originally referred to a child born in that days of Hezekiah. If it was appropriate to call that child “God” then why would it be inappropriate to call Jesus “God”?