I will tackle the question, assuming we talk about a beginner developer without much professional experience.

jQuery was created to simplify DOM manipulation through easy to use, cross-browser API. It may be tempting for a beginner to learn jQuery, because the library makes working with the DOM so pleasing:

jQuery:

$(el).fadeIn();

Vanilla:

function fadeIn(el) {
  el.style.opacity = 0;

  var last = +new Date();
  var tick = function() {
    el.style.opacity = +el.style.opacity + (new Date() - last) / 400;
    last = +new Date();

    if (+el.style.opacity < 1) {
      (window.requestAnimationFrame && requestAnimationFrame(tick)) || setTimeout(tick, 16);
    }
  };

  tick();
}

fadeIn(el);

For everyone new to JavaScript, the second example will look confusing and bewildering. But this is only because of the abstraction level jQuery provides. You can think of it as an iceberg - you see only the top of the iceberg, the small, beautiful part of it. But beneath it, there is an enormous piece of ice, hidden from our eyes.

My answer to your question will be no, I won't recommend a beginner to learn jQuery. And here is why.

Learn the basics

You possibly heard this a million times. Before jumping to any framework or library, learn the fundamentals. Learn pure JavaScript. It's OK to learn ECMAScript 6 in the process, it's actually a good idea to do so, but don't start learning React, Angular, jQuery or any other before having a strong foundation.

I've written a post about learning JavaScript, explaining how exactly my learning journey went

Marco Alka wrote another post about vanilla JavaScript vs JS libraries, which you should definitely check.

I have been tempted, more than I should, to start learning some new, awesome and promising library, but I resisted the temptation. You know why? Because libraries and frameworks fade, but knowledge doesn't.

Learn the fundamentals, have a strong foundation, become prolific JavaScript developer and just then think about what tool to learn.

Performance

jQuery ease of writing comes with its cost. Remember the example from before? The JavaScript example is 93% faster than the jQuery equivalent. Here is the proof - jsperf.com/fadein-to-native/1

Almost everything jQuery does, JavaScript does it faster. It may look that you are writing more code when using vanilla JavaScript, but that is not entirely true. Let's see how jQuery's fadeIn method looks like under the hood:

function (speed, easing, callback) {
    return this.animate(props, speed, easing, callback);
}

Well, that doesn't seems too bad. One line of code. But what's inside this.animate ?

function (prop, speed, easing, callback) {
    var empty = jQuery.isEmptyObject(prop),
        optall = jQuery.speed(speed, easing, callback),
        doAnimation = function () {

        // Operate on a copy of prop so per-property easing won't be lost
        var anim = Animation(this, jQuery.extend({},
        prop), optall);

        // Empty animations, or finishing resolves immediately
        if (empty || dataPriv.get(this, "finish")) {
            anim.stop(true);
        }
    };
    doAnimation.finish = doAnimation;

    return empty || optall.queue === false ? this.each(doAnimation) : this.queue(optall.queue, doAnimation);
}

Now that is something! You can see how much more operations the fadeIn method does, compared to our JavaScript example. This is where the difference in speed comes from.

The Library Trap

I like to call a person who invest most of its time learning a library or a framework, a "tool prisoner". I've been a tool prisoner once in my life. I started learning Backbone.js. It was a promising framework, used by several big companies. I've invested 3-4 months learning it. Right now, I cannot monetize my time learning Backbone anywhere. As of 2017, Backbone is on a steady decline. Here is the global interest drop:

Looking from the financial side, these 4 months were tragic. On the contrary though, I have learned a valuable lesson - libraries and frameworks fade, fundamental knowledge doesn't.