Low quality on Dimension export

if you rendor that model at that scale with that Uv map in any other rendor system i.e, Blender etc then yes results will be more or less the same as Dimension

if you take any image and blow it up then the result will always be bigger pixels but if you take a bigger image and scale it down then pixels become part of the background image... the model you are using is basic water drop i.e, designed to be small

you scaled the model up too high... I would ask Dimension to export the model then reimport it and generate a new Uv map

another option would be to generate the new Uv map in Blender but for something so basic Dimension should be fine