Not that it's Hull's fault, but one single Cup for a team that probably had the most high-end talent in the 60s can be seen as an underperformance. Also, despite the Cup, 1961 wasn't his strongest playoff showing; Pierre Pilote actually led the Hawks in scoring during those playoffs.
Taking a closer look:
In '61, the 3rd place Hawks win the Cup as the two best RS teams are upset in the SFs. The Hawks has been moving into the 3rd best team slot for the previous two seasons but were expected to challenge for the Cup. Hull has an impressive playoff, but Pilote is the clear Smythe winner.
In '62, the 3rd place Hawks reach the SCF, arguably another over achievement, with Mikita having one of the era's best playoff runs, Hull is very good with 8 goals in 12 games.
In '63, the 2nd place Hawks lost to the 4th place Wings in the SF, not a really an upset as the Wings were only four points behind in the standings. Hull has his best playoff run of his career with 8 goals in 5 games.
In '64, the 2nd place Hawks lose again to the 4th place Wings in the SF, but is an upset and really the first time you start to question Hull, who had two goals and seven points, and the Hawks,
In '65, the 3rd place Hawks got revenge on the 1st place Wings in the SF while Hull had one of the era's best single series performances putting up 8 goals and 13 points in seven games. They lost in 7 to the 2nd place Habs with Hull getting 4 points but still leading his team in scoring.
In '66, another disappointing SF loss to the Wings, Hull had 2 goals and 4 points while Mikita was worse.
In '67, the Hawks finished in 1st for the first time in the Hull/Mikita era and had the highest RS goal total in NHL history. They were upset in SF by the Leafs with Hull being solid with 4 goals as Mikita notably underproduced again.
In '68, the Hawks fall to 4th as their offense and defense both regress. They lose to the #1 Habs in the SFs. Hull is solid offensively.
In '69, they fall farther back in the standings and miss the playoffs.
In '70, they rebound to finish first in the league with a notably strong defense. They lose in the SFs to the Bruins. Hull is great in the QF and shut down with no goals in the SF.
In '71, the 3rd place Hawks reach the SCF only to lose another seven game series to the Habs, an especially bitter one as they were up 2-0 in Game 7. Hull is great throughout the playoffs.
So from the 60/61 season to the 70/71 season, the Hawks were the 2nd best regular season team, had the 2nd most playoff wins, and had the 3rd best winning % in the playoffs, slightly behind the Leafs,
They had 1 Cup and 3 other SCF appearances.
The Habs had 5 Cups in 5 SCF appearances.
The Leafs had 4 Cups in 4 SCF appearances.
The Wings had 4 SCF appearances.
Hull had 28 points and 13 goals in 26 SCF games while being the better playoff performer than Mikita.
That the Hawks did not win at least one more Cup is certainly up there with notable other team disappointments like the OV-era Caps but Hull's individual play is above critique, IMO.
He doesn't lose any points for his playoff resume but does not gain anything unlike Beliveau, Richard and Crosby.