Momentum is not evil, despite what you may have heard. In fact, it can actually be used to benefit our training in some situations when applied appropriately. The thing we should be concerned with is HOW that momentum was generated.
Was it created by a hard contraction of the muscle we are trying to train?
Or was it created by a compensation of other muscle groups that are simply taking tension away from the target muscle(s)?
Creating momentum by compensation or unnecessary movement can be counterproductive (like when you see people throw their torso backward to initiate a back exercise or swaying forward and backward in a DB lateral raise). It results in needing to do more sets and reps to get the same amount of volume of work for the target muscle. It is inefficient and ultimately creates more systemic stress than necessary to recover from in most cases. There can be a time and place to generate momentum in this manner, but certainly not all the time.