According to its molecular structure, CH3-o-CH3 is a polar molecule, because its O atom contains two lone electron pairs, which leads to the asymmetry (angular arrangement) of two methyl groups. The so-called methyl ether is a nonpolar molecule because its polarity is quite weak.

The following is an abstract of the article in Journal of Xi Jiaotong University (the key words in the following words are easy to find):

Liquid dimethyl ether is a polar solvent, and its polarity is caused by the oxygen atom in the middle of the molecular chain. Dimethyl ether is a weakly polar organic solvent because it has no polar molecular head.