Indeed, though I've heard that you can use Wabbajack to fix most followers. I've also heard that essential followers, like Mjoll the Lioness, cannot be fixed on consoles using Wabbajack because it will never change their form, only doing damage.
So if you want to use Mjoll at her best (level 40 I believe), apparently you would have to wait until your character was level 40 or higher before visiting Riften. Very lame.
Actually there is a way of resetting Mjoll without the Wabbajack, and that is to send her back to her home and wait 30 days. If you married her, she will return to your home I believe (haven't married her so I don't know) and player homes never reset, so she will never get updated either. This is why housecarls cannot be updated through resetting a cell, since player homes never reset. So basically don't marry her until you're sure she's updated to level 40.
On another note, I believe there were a bunch of stealth fixes in 1.5, stuff which they just didn't tell us. Because my Lydia handled a spriggan on her own today, a feat which totally astonished me. So perhaps they have already fixed followers levelling. Remember that to see this change, you will have to actually advance a level yourself to refresh your follower, or use the wabbajack/sending home method, which might explain why people aren't seeing this fix yet, but I do believe it is in game from my gut feeling about how improved Lydia is.