。
She's just putting on a show of being disappointed, you know.
Literal
She [topic-は] [just-ただ] disappointed-do [show by doing-て見せている] [it's just-だけだ] [you know-よ].
The auxiliary ~て見せる has two related senses: 'do X to show / demonstrate' (literal display) and 'put on a show of X-ing' (performative). Here it's the performative one — getting に gives the meaning 'is making a display of being disappointed,' i.e. her disappointment is theatrical, not real. ~ているだけ adds 'is just doing,' and the sentence-final よ asserts confidently to the listener: 'I'm telling you, she's faking it.' がっかりする is itself built on the 擬態語 がっかり (the deflated, sinking feeling of letdown). The whole sentence reads as an in-the-know aside between two observers.