Giving her bribes does indeed just cause her mood to go through the partially-randomized mood swing logic, yes. It doesn't put her automatically in a good mood. However, the mood determinations are not just based on randomness. There is what we call a "grab bag" of mood entries (think of it like a raffle with X number of tickets for each mood type), and how many raffle tickets each mood gets in the bag is determined by a number of factors.
Broadly speaking, here's how that works:
Warlike automatically gets 10 tickets.
-- It also gets 1 ticket for each 0.01 point of racial compatibility it has on its planet less than 1. So 0.94 compat on its planet is 6 more tickets.
-- For all living races that it has a negative attitude towards, it gets 0.25 tickets for every point below 0 attitude.
-- For each hive queen that has been assassinated on that planet, plus one, times 2 overall, there are that many tickets subtracted (so 3 assassinations would be 8 tickets lost at that planet).
Neutral automatically gets 3 tickets.
-- For all living races that it has a positive attitude towards, it gets 0.4 tickets for every point above 0 attitude.
Cooperative automatically gets 1 ticket.
-- For all living races that it has a positive attitude towards, it gets 0.1 tickets for every point above 0 attitude.
-- It also loses 0.1 tickets for each 0.01 point of racial compatibility it has on its planet less than 1. So 0.94 compat on its planet is 0.6 fewer tickets.
-- For each hive queen that has been assassinated on that planet, plus one, times 2 overall, there are that many tickets subtracted (so 3 assassinations would be 8 tickets lost at that planet).
Defensive automatically gets 1 ticket.
-- For each hive queen that has been assassinated on that planet, plus one, there are that many tickets added (so 3 assassinations would be 4 tickets added at that planet).
-- Additionally, looking at every planet owned by the race, if there are bad economic or environmental situations, then they also contribute tickets. I didn't dig into the logic here.
Brooding automatically gets 10 tickets.
-- For all living races that it has a positive attitude towards, it loses 0.1 tickets for every point above 0 attitude.
-- For each hive queen that has been assassinated on that planet, plus one, there are that many tickets added (so 3 assassinations would be 4 tickets added at that planet).
Contemplative automatically gets 5 tickets.
-- For all living races that it has a negative attitude towards, it loses 0.1 tickets for every point below 0 attitude.
It then runs the raffle and sees what the result is. If you keep popping up warlike, then that may just be the RNG hating you, or it is more likely that relations between the Thoraxians and the other races really stink. Ironically, killing some hive queens a few times over on that planet will actually help make them less warlike over time (put them in their place!), though there are other negative ramifications from that in general of course.