not necessarily. Walker and TT are still quality NBA players. So's Smart who's the third option for a vet to be traded. They're not players that have to be moved as salary dumps so if picks/youth are included in the deal it's likely because we're getting back a better player than who we're shipping out.
Walker is a useful NBA player when he's able to play. This year he wasn't able to play back to backs, and he's already missed a game in a very short playoff run.
Walker has also shown that his scoring efficiency is very inconsistent now, and he has problems defensively against opponents with big guards / smart enough to pick on him.
He's also got an enormous contract with multiple years left.
Kemba is not a positive asset and will almost certainly require additional assets attached to move him.
Thompson is a decent player, but his play and production this season has by and large not been up to the level of what he's making. He's overpaid, but will also be an expiring contract next season.
I don't think it would be necessary to attach additional assets to Thompson to move him. However, I don't think the Celts would get any player or pick of real value in return unless they were willing to take back a worse contract (ie multiple years remaining), or include other assets in the deal.