Is 1-methoxy-2-methylpropane a positional isomer of 2-methoxy-2-methylpropane?
2-methoxy-2-methylpropane is a synonym for MTBE (Methyl tertiary-butyl ether). The name "propane" is a little bit missleading, because it comes from the standardized IUPAC nomenclature system, which names the  ether-oxygen together with the smaller alkyl group an "alkoxy" group with the suffix "-oxy" (in our case: methoxy) and takes the larger alkyl part as its base identifier (here propane) when describing ethers. So MTBE is referred as an ether with all its chemical properties. 

Now to the isomerism: Structural isomers have the same chemical formula but different constitutions (arrangement of atoms, bonds, groups). So they definitely must be constitutional isomers. 

Constitutional isomerism is divided into 5 subcategories: 


Chain isomerism
Positional isomerism
Functional isomerism
metamerism
tautomerism


As you already mentioned: they are metamers. Thats because different alkyl rests are attached to the same functional group (ether). This is the first reason why they are not likely to be positional isomers. And the other reasons might come from the "moving substituent" (alkoxy group), which has a carbon atom (the carbon structure has to be constant within positional isomers, therefore moving the -OCH3 could mean a difference) and is no functional group.