Many people tell me that dmt tastes like mothballs. does washing xtals remove some of that particular taste? just wondering if this distinct flavour is particular to the pure Substance or just the contaminants (lye etc.)

I can't say I'd agree with that. You can always sandwich your DMT between some herbs, like Mullein or some peppermint, or even infuse the DMT into the herbs.

Also, personally I find the smell of DMT comes from the bark itself and the oils transferred in the extraction process. I'm not sure if pure DMT would even smell.

Burned DMT is horrible. DMT properly vaporised is smooth and doesn't taste of much.

that taste/odor is characteristic of the structure of the indole structure in DMT, specifically its aromaticity (in chemistry terms). it's structurally similar to napthalene, except it's a heterocyclic structure containing a nitrogen instead of all carbon-hydrogen bonds.
it has nothing to do with impurities. I have ACS-grade tryptamine HCL (> 99% pure), it smells similar.