Accelerometer alone should be more than enough for such task as it has been used widely in activity recognition studies. If I were you, I would start with the accelerometer only and collect data from experiments to what you consider "smooth walking" for your robot. You could also try extracting time/ frequency features from the accelerometer data and use that to compare between desirable vs undesirable walking.

Look at this,

https://github.com/TKJElectronics/KalmanFilter

Best wishes