If you're targeting her directly, yes.
However, consider the following:
-Left and Right are named characters and thus targetable with assassination attempts.
-Neither of them has any bonuses against preventing their OWN assassination.
-Littlefinger, if he doesn't do anything else with his intrigue abilities for a turn, can fire off a whopping three assassination attempts, plus use any assassin characters in his army- and he can actually afford Faceless Men, unlike most.
-Assassinations are resolved in the order of the assassinating player's choice.
-With her bodyguards gone, the Queen of Thorns has only her own intrigue score to defend against assassins, which a Faceless Man can ignore.
What this adds up to is that Littlefinger can assassinate the Queen of Thorns effectively and reliably, but only if he's willing to throw an entire turn's worth of actions at it. That's more than anyone else can say- but on the other hand, Littlefinger's personal intrigue actions are basically the most precious resource available to his player, and spending them all killing off Tyrell's only decent intrigue character means not spending them on something which would usually be more useful.
On the other hand, once the Queen of Thorns is dead the Tyrells become Littlefinger's bitches, so I can see it being worth the effort under some circumstances. It's options like this that make GoT mode so entertaining.