Top 30 all-time in the following categories:
OBP: 21st.
SLG%: 25th.
OPS: 15th.
HR: 18th.
BB: 9th.
RBI: 22nd.
This is all time.
Usually likely HOF are held to the "Decade of dominance" standard, which means a guy needs to be absolutely dominant at least across what would be considered a decade of "Prime years".
Frank Thomas' decade of dominance:
1990-2001: R:1091 H:1770 2B:364 3B:10 HR:348 RBI:1193 29 BB:1198 SO:847 BA:.319 .OBP:438 SLG:.577 OPS:1.015 OPS+167 TB:3198 .
That is one of the most dominating stretches of baseball in history.
Rest of the way:
BA:263 OBP:.378 SLG:.509 OPS:.887 OPS+:131
Still upper echelon production,
Baseball Reference's HOF monitor ranks him in the following manner.
Black Ink Batting - 21 (98), Average HOFer ≈ 27
Gray Ink Batting - 200 (39), Average HOFer ≈ 144
Hall of Fame Monitor Batting - 194 (45), Likely HOFer ≈ 100
Hall of Fame Standards Batting - 60 (32), Average HOFer ≈ 50
He was one (if not the) most dominant player in the 1990's. The stats reflect this. The only knock on Thomas is his DH time, otherwise, he's a sure first ballot.