Not a violinist but I used to be a professional double bass player and the issues are somewhat similar (actually worse on violin than double bass because the gauge is shorter and therefore the margin for error is less).

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).