Is it really a sin to miss Mass on Sunday?

Yes. "The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation... Those who deliberately fail in this obligation commit a grave sin" (CCC 2181).

Why it is that serious. The Third Commandment orders us to keep the Lord's Day holy. For Christians the Lord's Day is Sunday, the day of the Resurrection, and the way the Church has kept it since the apostles is the Eucharist (Acts 20:7; CCC 2174-2178). To skip Mass by choice is to say that something else matters more than Christ's sacrifice offered for you. That is why the Church counts it as grave matter.

What excuses you. The obligation binds only when it can be fulfilled. You are not bound when you are sick, caring for a sick child or parent, unable to travel, working a job you cannot leave (nurses, first responders), or in similar genuine impossibility (CCC 2181; Canon 1248 §2). "I was tired" or "we had a game" do not count. A Saturday evening vigil Mass fulfills the obligation. Watching Mass online does not, though it is a good practice when you truly cannot attend.

If you have missed. If you skipped without a serious reason, confess it before receiving Communion again. Then come back. The Church has kept Sunday for two thousand years because we need it: "Without Sunday we cannot live," said the martyrs of Abitene in 304. The obligation is not a burden imposed from outside. It is a mother making sure her children eat.