Of course it is!
This begs the question "But the Church go along just fine for 1950 years without having to believe this! Why do we have to believe it now?"
Answer: becasue we now know that the Assumption of Mary is revealed by God.
Put another way: disobedience is the sin. Once the Church has made a solemn judgement that something like the Assumption of Mary is revealed, the sin is in rejecting the authoritative judgement of the Church. Prior to 1950--there was no authoritative judgement. Catholics were free to form their own opinions about the Assumption. After 1950, that was no longer the case.
Again, sir, not rocket science.