anticlimaxnoun [ C or U ]
uk/ˌæn.tiˈklaɪ.mæks/us/ˌæn.t̬iˈklaɪ.mæks/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.
Coming home after a trip somewhere is always a bit of an anticlimax.
Even when you win a match there's often a sense of anticlimax - you always feel you could have played better.
anticlimactic
adjective uk/ˌæn.ti.klaɪˈmæk.tɪk/us/ˌæn.t̬i.klaɪˈmæk.tɪk/
There was so much publicity and hype beforehand that the performance itself was a touch anticlimactic.