Currently the creaking heart is inside the 'Spawn Eggs' creative mode tab next to the two spawner blocks, with the spawn egg for the creaking also already appearing in that tab (notably it does spawn a slightly different 'type' of creaking mob).

While the block is indeed involved in spawning the creaking mob, it's placement in the creative inventory is awkward for the following reasons:

1. The creaking heart (a block) is inside the 'Spawn Eggs' category, despite not being a spawn egg item, or being directly tied with spawn eggs (spawners are blocks, but are directly involved with spawn eggs). This makes the inventory tab's name no longer accurate to what is inside it.
2. Should it be decided that the creaking heart would stay inside this category, its placement is awkward because the creaking heart is not the only item required to spawn the creaking. A pale wood/log item is also required, meaning that all resources required to summon the mob are not included inside of the tab.
3. The creaking heart block is a functional block, meaning its placement could be unexpected.