QIIME 2 does not have a method for making distance heatmaps.
If you are proficient with python, you could view as a heatmap easily with scikit-bio. Something like this:
import qiime2
import skbio
dm = qiime2.Artifact.load(distance-matrix.qza).view(skbio.DistanceMatrix)
dm
[the output of dm will be a distance matrix heatmap]