"Big deal" can be used to talk about something which had been exaggerated or that we don't think is so important.
For example: Your friend tells you they just got a new phone, but you are not really impressed. You reply, "Big deal, it's just a phone...."
Or, your friend surprises you for your birthday with a huge cake and party with 50 friends. They made a "big deal" of your birthday. (You think they did too much for this event.)