Ironically, ISIS had the same problem that the U.S. did in Iraq: An inability to politically and militarily secure the territory it captured. If you can't hold the territory for the long-term, militarily and politically, you are wasting lives and money in capturing it.
Regarding the U.S.: If we talk about what 'could be' or 'maybe' or what we 'imagine', as the parent comment does, we can make any claim. Based on what the military and experts have consistently said and on the facts on the ground: It's the same plan, working as designed; ISIS' loss of territory did not coincide with Trump taking office, it began in 2016.
> ISIS also grew on his [Obama's] watch
That has no implications for Trump's effectiveness.
> we knew about it [ISIS] and maybe even allowed it grow, in order to destabilize Syria.
Starting form the fundamentals of international relations (as I understand them), as the global, status quo superpower the U.S. wants stability globally, which protects the U.S.-led international order; ISIS is a threat to global stability. The U.S. has particularly supported stability in that region, to secure the global oil supply (essential to global stability and the U.S. economy) and also to protect the region's status quo U.S.-supported power structure, based in recent decades on the Saudis, Israel and Egypt. The U.S., by its actions, clearly prefers Assad to ISIS: The Americans even have allied in places with enemies including the Iranians (including in Iraq), Russians, Assad, and even other small fundamentalist groups against ISIS.
That doesn't mean ISIS took the U.S. by surprise, however. ISIS began in Syria, outside of the U.S.'s sphere at the time, and then spread to Iraq. The U.S.'s options in Iraq at the time were limited. The Iraqi government refused to allow U.S. forces in the country (that was the deal agreed to by Bush and executed by Obama, withdrawing U.S. forces). Also, the Iraqi army may have been expected to put up a fight instead of simply fleeing when faced with what were (IIRC) inferior forces.