It could be either - or a combination of both faulty guitar set up and playing technique that needs refinement.
Wheat's answer is spot on. I would like to point out that it is normal and very common (pretty much the rule) that beginning guitar players experience buzzing or muting (or both) on many chords. It takes repetitive practice to master the proper technique and to build the necessary finger strength to cleanly fret all strings in various chord shapes.
Be sure you position your hand to form the chord so that you can apply sufficient pressure to each fretted string in the chord. You should strive for a finger tip that is perpendicular to the fretboard to be sure your fretting finger does not unintentionally contact an adjacent string and mute it (unless the chord shape calls for an adjacent string to be muted intentionally). It may take quite a bit of time and practice before you will be able to play all chords cleanly without buzzing.
Here is a simple test to determine if it's your evolving technique causing the buzz or the guitar. First, determine which string fret combinations are buzzing when you play a chord and try isolating that string/fret note by playing only that string on that fret with one finger. If it only buzzes at that fret on that string when you play a chord that includes that note, it is more likely to be in your technique. If it buzzes even if you play a single note there - your guitar probably needs a set up.
Either way, a proper set as Wheat suggested, up will help make your guitar easier to play without technique induced buzzing.
One thing I will add regarding set ups. Your string gauge will affect your guitar's playability (how comfortable it is to play) - and a set up will need to be done specific to the string gauge used on your guitar. Most acoustic guitars like your Yamaha, come with what I call heavy light gauge strings (the heavier end of what is still considered light). The gauge of strings is often defined by the gauge of the high e-string. Thus .013 inch high e - would indicated a medium gauge set, .012 a heavier light gauge (common on many new guitars) - .011 a custom light or medium light and .010 an extra light gauge.
Most new guitar's (if any set up occurs in the factory or at the dealer) will be set up for whatever gauge string the guitar ships with. If you do decide to have your guitar set up by a professional, you might consider switching to a lighter gauge string - until you build your finger strength and perfect your fretting technique. Lighter strings on a guitar properly set up for them, will make your guitar easier to play without buzzing. Buzzing (in contrast to muting) that is the result of un-perfected technique, occurs when you are not able to apply enough force to the string to make it play cleanly. Lighter strings tune to pitch with less tension and therefore less pressure will be required to cleanly fret a note. Learning to play has plenty of frustration inherent in the process - so anything you can do to remove some of that inevitable frustration will increase the likelihood that you will experience the joy of making progress and continue the journey.
Your guitar technician will be able to determine the lightest strings the guitar can handle without changing the nut or other components. You will lose some volume with custom light or extra light strings, but I would prefer a little less volume with no buzzing than loud - but buzzing. At least until my playing skills improved enough to switch to a heavier gauge and still play without fret buzz.
If you simply switch to lighter strings without a set up, the lighter strings may contribute to guitar induced buzzing. That's because the lower tension on the lighter strings means the oscillation envelope will be wider and your guitar may not have sufficient relief to provide clearance between string and fret. If you did the test I suggested above and determined that you only get buzzing on certain chords and not isolated notes, you could try to switch to lighter gauge strings without a set up if there is enough relief already between frets and strings. The string most likely to buzz - when switching to a lighter gauge - is the low E string which has the widest oscillation envelope. If you buy a set of extra light strings, and change the low E string first, then tune to pitch and fret each note one at a time all the way up the fret board and get no fret buzz, you should be fine to install the balance of the set without fear of guitar induced fret buzz.
But one last time, a good set up by a qualified professional will ensure that your guitar is playing optimally and with minimal stress on your finger muscles, regardless of what gauge strings you choose to use! Get one done if you can.
Good luck and enjoy your journey!