I suppose what you mean is, is such a length typically a non-integer. (Any number can be written as a decimal, like 1.0! Any number can also be written as a fraction.)
The answer is that the length of anything at all can be an integer, or not. A length might be something like 12, or 5.2, 1/3 = 0.33333..., or √2 = 1.4142... (that is, integer, terminating decimal, repeating decimal, or irrational). An arc might have any of those lengths, too.
In the real world, if you measure any length, it will be more likely to be non-integer than integer, but there is no reason it has to be. It's just that, in any given size range, there are more non-integers than integers to choose from. And since nothing can be measured exactly, even if I measured it as 12 cm, it might really be 12.001 or something.