Ableton's Learn Live series walks through the first steps of cleaning up a MIDI bassline in Live 12's Note View, covering three quick but essential editing moves: quantization, scale visualization, and pitch correction.
Quantizing snaps notes to the grid via Command U (Mac) or Control U (Windows), and grid resolution can be adjusted from the top-right menu. Pressing K highlights the notes belonging to the clip's scale, while F and G toggle Fold Notes and Fold Scale to hide everything outside it.
The Fit to Scale button in the Pitch and Time Utilities tab nudges notes into the clip's key without manual correction. With no notes selected it applies to the whole clip, but you can also target a specific selection if only part of the bassline needs fixing.