Thanks for your answer but I'm not sure the problem comes from the export function.
I made some other tests and here is another example of my problem.
I have import 3 images in XD, they are at the same size in XD (50*67px) but their original size are different. I wrote the resize value above each image (their originals dimensions are : 50*67 for the left image, 100*134 center, 226*307 right).
We can see that the left image is sharp and the right one is a bit blurry.
The conclusion I would make is the more I downsizing the images, the more the blur effect appear.
In photoshop, we have "Smart objects" function which works well for this kind of case with no blurry effect.
Do you think this is normal or could it be fixed ?