She's not underpowered, she's underplayed, learn the difference.
I blame the design team for her kit.
Look at Cho'gath, his kit is brillant, especially his ultimate, OM NOM NOM and the way he grows as he omnom more.
Sejuani? Quite plain HA. She slows and slows more, I think we get it.
I especially miss some kind of interaction with her boar. She rides a freakin boar, and yet, you could remove the boar, no one would notice, the hell.
How to fix?
Give her a second passive.
The boar is now alive, and OMNOM and sidekicks all around as Sejuani goes and charges into battle.
Not only that would be so ****in awesome, but that would give her some more dps (similar to Blitzcrank's lightning bolt passive).
Good or good?
Edit: you don't have to make it complicated. Every 3 or 4 attack, her boar bites the target once (+xx damage), as simple as that. That would even help her jungle, how awesome.