'귀여웠던데' is incorrect grammatically because '귀엽다' is an adjective, whereas '합격했던데' is correct since '합격하다' is a verb. Adjective does not connote the concept of 'grammatical' completion. 던 is the past tense implying a simple or continuous state. 데 is used when you experience and tell(reporting) something to other(s). As a set of '던데', its meaning varies according to its intonation, a simple statement or exclamation.

던데 is used when you found that something was contrary to my previous opinion/expectation or opinions/expectations of others. 이미 하던데 = (I found that they) were already doing it. 았/었던데 is used when you found that something was contrary to my previous opinion/expectation or opinions/expectations of others. And it is certain that the situation is finished. 이미 했던데 = (I found that they) already done it. 그 애가 귀엽던데 (I found that) she was cute. (I'm not sure that she is still cute or not). 그 애가 귀여웠던데 (I found that) she had been cute. (I'm sure that she is not cute any more).