The Greek word used in Galatians 3:1 is ἀνόητοι (anoētoi).
Ὦ ἀνόητοι Γαλάται, τίς ὑμᾶς ἐβάσκανεν, οἷς κατ’ ὀφθαλμοὺς Ἰησοῦς Χριστὸς προεγράφη ἐσταυρωμένος;
There are 2 meanings according to Concise GREEK-ENGLISH DICTIONARY of the NEW TESTAMENT. This is “foolish” or “ignorant.”
NRSV uses “foolish”. The Greek word, ἀνόητοι can also be read in Luke 24:25 Jesus used it with his two disciples.
καὶ αὐτὸς εἶπεν πρὸς αὐτούς · Ὦ ἀνόητοι καὶ βραδεῖς τῇ καρδίᾳ τοῦ πιστεύειν ἐπὶ πᾶσιν οἷς ἐλάλησαν οἱ προφῆται ·
Now, think about the CONTEXT (not simply the word).
St. Paul describes the Galatians as foolish and asks them rhetorical questions to contemplate on their past and present lives. If they started well with the Spirit (Galatians 3:3) and continue experiencing the Spirit in their lives (Galatians 3:5), they are foolish by believing in another discourse that concludes by subjecting them to the Law.
So Foolish is correct. They are doing something they should not be doing.
νοος means “mind, thought.” ἀ-νόητος means “no thought, not thinking, not thought-out.”
St. Paul means they’re not thinking. So “foolish” is much better. “Stupid” only as emotionally exasperated.