What happens if a creature's ETB would bounce Thalia, Heretic Cathar?
Yes, this is correct. Thalia's ability is a so-called replacement effect

614.1. Some continuous effects are replacement effects. Like prevention effects (see rule 615), replacement effects apply continuously as events happen-they aren't locked in ahead of time. Such effects watch for a particular event that would happen and completely or partially replace that event with a different event. They act like &quot;shields&quot; around whatever they're affecting.
614.1d Continuous effects that read &quot;[This permanent] enters the battlefield . . .&quot; or &quot;[Objects] enter the battlefield . . .&quot; are replacement effects.

which doesn't go on the stack. Only when the new creature actually enters the battlefield (tapped), its enter the battlefield ability will trigger.