True.
Try this analogy on for size: “bad liturgy” is like a poor NFL rulebook. If it is poor, it poses obstacles to playing the game at its highest level. It makes officiating a nightmare. It keeps talent from expressing itself. It leads to technical developments in the sport that unfairly exploit unintended loopholes in the rules.
If the rulebook is poor, top level play is not impossible. The winner at the end of the season is still the NFL champion. Teams that win and are within the rules are not cheaters. But even when the game is played at the top level, a desire to improve things remains when the season ends and is reviewed. Also, sometimes even a good change in the rules needs adjustment to correct unintended consequences. It is not that the change needs to be thrown out, but it does mean that the rulebook needs a re-write to make the change that was actually intended the first time around, without the unintended side-effects.
In this sense, yes, a Pope can promulgate a less satisfactory liturgy, even a bad one. Using the football analogy and applying it to the Church’s case, though, what he promulgates still defines what football is. Defining liturgy is one of the Pope’s roles. I don’t think any of the current liturgies are bad, but I think the Church’s desire to keep improving the GIRM are very worthwhile.