The new guy couldn't carry it nearly as well as Mako could.
There's a semi-good solution, though; have Aku be silly the entire season...until the final battle where he gets annoyed enough at Jack's allies attacking him to get serious and revert to the evil rampaging tangible evil he's supposed to be.
Having Aku kill the Scotsman in episode 5 or 6 was really blowing the wad early. The ghost Scotsman doesn't even do anything until the last episode anyway.
The episodes-long joke about Scaramouche finding his way to Aku wasn't funny enough to pull its weight.