Yes it should be, somewhat anyway. Primarily batteries wear out through use, both charging and discharging - if your phone is plugged in but not charging, then your phone is running off mains and the battery isn't being used. However, batteries also wear out over time, albeit much more slowly. Manufacturer's typically recommend keeping batteries in long term storage at around 70% to minimise this effect. If you're always plugged into mains at 100% charge, then you're not following this storage recommendation. Leaving it at 100% won't kill the battery, but its capacity will go down quicker than if left at a lower charge.