I've read that one cause of danelions could be to much acidity. Too lower the acidity, can I put lime directly on top of my lawn?

You sure can.
Use either pelletized or powder.
I don't know about the dandelion part, though.

If you're thinking your soil needs amendments, it's best to get a soil test first. If you have a state university, they often have an extension service that offers soil testing.

Dandelions grow because seeds land in your lawn.

Dandelions grow in a wide variety of soils, but grow most bestest in the same soil that grows a good, healhty lawn with a soil pH between 6.2 and 6.8, so it is really important that you get a good, reliable soil test before contemplating applying lime to your soil. Your state University's USDA Cooperative Extension Service, if you live in the USA, can give you more information about this, but the best way to control any "weed" in your lawn is to grow a good, healthy lawn.

Mertz, if you read articles on 'starting a lawn', or in 're-seeding' or 'fixing' a lawn...you wil undoubtedly read about how lime is recommended to be added.

When you consider how much the average homeowner puts high nitrogen fertilizer on his lawns every spring, you can see that doing so causes the lawn to take on a high acidity.
Using lime to raise the pH of such lawn is recommended by all authorities on grasses.

If you feel you also apply such amounts of nitrogen fertilizer to your lawn....
go ahead...apply lime but first read about how much should be applied and when the best time to make the application should be made. The bag might also have such directions for use.

Many people just add lime every one or two years routinely figuring the lawn can use the chemcial to bring up the pH. This is haphazard but as long as you don't over-apply, and keep check on your lawn's health, not much harm is going to come to it.

It is true though, adding chemicals to a lawn while not knowing what the chemical might do for it is taking a chance your lawn might suffer.

In the local garden centres/nurseries, they sell a non-professional tool to measure acidity.
You might consider using such and taking samples from many areas of the lawn before you apply lime.

As far as dandelions, I don't think that weed has any particular preference for what it grows in.
If your adding lime was an attempt to control this broadleafed weed I would suggest you put it out of mind. After all, besides, where you see dandelions you will also see many other broadleafed weeds.
A general spraying of a broadleafed weed killer before the dandelions go to seed will fix the problem for a time. That is, until the seeds blow in from your neighbors.

We lime our yard about every three years.
Tom, the local lawn care company owner, just did ours a week and a half ago. Since he knows our yard, he recommended it be done this year. We trust him and his advice.
He does apply sufficient nitrogen fertilizer so we do trust when he suggests liming.

I have not found the very cheap methods sold in the local garden centers to be a very reliable measure of much of anything. Many people have purchased a so called pH meter and found that when they tested their soil, a baking soda solution, and some vinegar, the meter gave them the same reading indicating that that meter was not working since a baking soda soluton should read somewhere around 8.0 while vinegar should read somewhere around 3.0.
A good, reliable soil test should always be done before putting down anything like lime so you know just how much to put down. Not putting enough will not do what it is supposed to and putting down too much will cause other problems that you do not want to have. No one can look at a soil and tell whether and how much of any nutrient that soil needs.