From Catholic doctrine: yes, by its own definition
If the question is asked from within official Catholic doctrine, the documented answer is clear: yes, it's considered a sin. Catholic.net describes it explicitly as "a very grave sin" for any Catholic who worships Santa Muerte, arguing that she is attributed powers that, under that doctrine, belong to God alone. You can read the full reasoning in our guide on what the Church says.
From real devotional practice: most devotees don't experience it that way
This is where it's worth separating institutional doctrine from the real religious life of millions of people. Many devotees consider themselves practicing Catholics — they go to Mass, go to confession, baptize their children — and keep a home altar at the same time. For them, there's no contradiction of the kind official doctrine points to; they see Santa Muerte as a guide or companion, not a substitute for God.
There's no single correct answer
This guide can't tell you what to believe — it can only show you that two distinct frameworks are at play: an institutional one (which considers the practice a sin) and a personal/popular one (which doesn't experience it that way). How to reconcile the two, if at all, is entirely up to you.
Related: What does the Church say? · Is she a saint? · Is it a religion? · Who is Santa Muerte? · History and origin