Is missing Sunday Mass a sin?

Yes. Deliberately missing Sunday Mass without a serious reason is a grave sin. The Church obliges Catholics to participate in the Eucharist every Sunday and holy day of obligation, and willful violation of this precept constitutes a mortal sin when the conditions for mortal sin are met.

The Scriptural Foundation

The early Church gathered on the first day of the week to break bread (Acts 20:7), a pattern that became the normative Sunday Eucharist from apostolic times forward.