Beginners do sometimes stick stickers on the fretboard, however these are generally not encouraged.
Firstly if you're looking at the fingerboard while playing a violin your neck would be in a really bad position, so it would encourage a bad habit. To learn to play in tune, you train your ear (so you know what in tune is) and your proprioception (so you know where your body needs to be in space to make the instrument be in tune).
Secondly, the difference in area between the note being beautifully in tune and sounding awful is really small, so in order for a sticker or visual cue to help your head and the instrument would have to be in exact alignment at all times and even then you may not be able to see. This is made much worse by parallax. When playing the violin even if you are looking at the strings, you are looking down the fingerboard towards the nut rather than across the fingerboard like in the case of a guitar, remember.
Thirdly, if you're looking at your fingers you're not looking at your music (which you need to when you're a beginner) or your conductor or fellow players (as you need to when you're actually performing).
I agree with the above but also add that any permanent markings would be problematic because wood expands and shrinks and therefore markings would not remain in the place.
That being said, on the rare occasion of a double bass solo in orchestra I would usually mark a starting point on my fingerboard with pencil.
It does require a good sense of relative pitch, which the student may need to develop first, but this is not unique to the violin. Singers and trombonists are obviously in need of a similar thing, as are theremin players, but slightly more surprisingly to those who don't play them are reed players, who have an incredible ability to play out of tune by simple adjustments of their lips around the reed, and recorder players, who can play horribly flat just by not blowing hard enough into the instrument (or too sharp, by blowing too hard).
Stickers on the fingerboard can help us when we don't already have the relative pitch sense, because you've got to learn it somehow and "go learn another instrument first" is not a very encouraging strategy!
There are actually products you can buy which are basically a sticker for the whole fingerboard laying out a fret pattern but without the physical attributes of frets, which would alter the sound of the instrument and reduce the ability to play slides, but I don't know anybody who's got one. It's not as big a deal as you might think before you start.
I say that as someone who, before taking up the violin, learned to play the viola da gamba in part because it has frets and this was far less scary (learning to use a bow was scary enough).
I still play both, and the playing technique on the viol is very definitely informed by the presence of the frets and their effect on the sound.
The issue is that the locations simply are not fixed. For one, there are different ways to tune a violin, and second, depending on what environment you play with and with what other instruments you adjust your pitch automatically accordingly. Since it is so very easy for the violin this is the instrument which adjusts to the others, the piano can't do that on the fly.
I also can confirm that it is much easier than it looks at first. Somebody said most beginners use markings, but the few other beginners I know, but also the violin teachers I know, don't use them (when I say teachers I mean for their students of course). I didn't think about it much when I started a few years ago (at well after 40, but it's fun and works out quite well), I never felt a need for it.
What you notice when you play is that it actually is pretty easy to adjust on the fly. You develop an ear for the right pitch (if you don't you may have a problem). In my experience you quickly place the finger in at least almost the right place, I doubt that any marking would help do it better when you are in the middle of a song, unless it's the slowest of slow. For the tiny differences that you need to adjust for markings are of no help because I would say you cannot hit them as accurately as would be needed anyway.
Even if I had markings, they would be useless. I would not look at them most of the time, besides, you are looking at quite an acute angle since you are holding the violin with your chin, so you would have to mentally adjust for that too! I think it may well end up being more of a hindrance than an aid.
A predecessor to the cello, the viol or viola da gamba, actually had more than markers: it had frets.
Have a look at one being played: https://www.youtube.com/watch?v=IP0YrMKZhkM