anticlimax (n.)
An event, period, or outcome that is strikingly less important or dramatic than expected.
The last chapter of the book was an anticlimax.
M-W
An event or experience that causes disappointment because it is less exciting than was expected or because it happens immediately after a much more interesting or exciting event
When you really look forward to something it's often an anticlimax when it actually happens.
Cambridge
anticlimactic (adj.)
Of, relating to, or marked by anticlimax
M-W
If an event or experience is anticlimactic it causes disappointment because it was less exciting than was expected, or happened immediately after a much more exciting event or experience.
There was so much publicity and hype beforehand that the performance itself was a touch anticlimactic.