American citizens are, of course, allowed to have such opinions, but we're not talking about a few citizens here - we're talking about what's considered one of the most influential political lobbyist organizations in US.
AIPAC is nominally funded by "individual donations", but the problem with that it is fairly trivial for a nation-state to fund things in this manner, so it doesn't really tell us anything. And then, of course, there's the question of personal connections: if you have a private person donating huge amounts of money to AIPAC, and they just happen to be heavily involved with the governing political party in Israel, I would consider them an agent of the state of Israel ipso facto even if they don't have such official position (just as I would consider, say, one of Putin's pet oligarchs a Russian agent if they donate money to an American political campaign in their private capacity).
It seems you think Israel has direct finance or control interest in AIPAC, do you have something to support that?