It is "perfectly reasonable" for Ukraine to strike targets deep inside Russia, in part to "degrade" Russia's military, says Richard Dannatt, a former head of the British Army. In an interview with RFE/RL's Georgian Service, Dannatt also says that given the slow pace of Ukraine's counteroffensive, Kyiv and its Western partners should prepare for a likely protracted conflict.
During a total war any reasonable military will prioritise destroying their enemies capacity to wage war. That typically includes prioritising munition spending on military targets.
Bombing a civilian city centre can be demoralising, but history shows that it primarily serves to harden your enemies resolve, because you are explicitly showing that you are willing to harm the civilian friends and families of those fighting or otherwise supporting the war effort.
The bombing of Hiroshima/Nagasaki is a prominent counter-example of this though, where the weapons used were so completely terrifying that they helped convince Japanese leadership that their entire nation could be wiped out if they didn’t capitulate. Still: there are strong arguments to suggest Japan would have capitulated anyway. Note that even though other bombing campaigns killed more people than the nukes, they didn’t cause a capitulation.