Does a proliferate effect let you add loyalty counters to a planeswalker?

Proliferate means you choose any number of players and/or permanents that already have at least one counter on them, then put one more counter of each kind that permanent already has on each chosen permanent (CR 701.27). Since planeswalkers enter the battlefield with loyalty counters and use them constantly, they almost always have at least one loyalty counter on them, making them legal targets for proliferate.

The key restriction is the 'already has a counter' requirement (CR 701.27b). You cannot use proliferate to place a loyalty counter on a planeswalker that currently sits at zero loyalty — but such a planeswalker would have already been put into the graveyard as a state-based action (CR 704.5i), so this situation is essentially moot in practice.

It is also worth noting that proliferate adds one counter of each kind that permanent already has. If a planeswalker somehow had both loyalty counters and, say, fate counters on it, proliferate would add one of each type.

Concrete example: You control a Jace, the Mind Sculptor with 2 loyalty counters on it. You cast Contentious Plan, which draws a card and then proliferates. You choose Jace as one of your proliferate targets. Jace now has 3 loyalty counters on it.