It's a bit early to assume a decline in sales. Day 1 sales are low but there are plenty reasons for that.
- Many big releases on the 18th.
- No marketing as opposed to hourly commercials for GTA5 and AC Unity
- It's not a day 1 title for those not interested in Create. If you wait a while you'll have tons of good levels to play.
LBP3 will get some more exposure once the newly created levels and ideas get some coverage. At least I hope the game sites will run another best of article some time.