Yes, non-Hispanic people should celebrate Dia de Los Muertos if they wish to do so. Celebrating holidays of other cultures is a good way to show respect for and interest in people of different backgrounds. Non-Hispanic people that have relationships with Hispanic people should celebrate the holiday in solidarity with those they care about.
The holiday of Halloween was appropriated from Celtic , Gallic and Aztec culture both. The pre Christian Irish, Welsh, British honored their dead like the Aztecs their holy holiday was called Samhain. So the face painting was similar to scare away evil spirits and ancestors were honored and people dressed as ancestors and meals were placed in front of them to honor the dead. Children had to guess the ancestor the person was dressed as if right they got a treat if wrong a trick would be played on them. This was the Goddess religion as well so white people did not steal this face painting idea. Christians changed it
Yes, non-Hispanic people should celebrate Dia de los Muertos, if they want to. In most cases, a non-Hispanic person is likely to celebrate the holiday in a different way because they are not used to the customs associated with the holiday, but that should not prevent them from celebrating. Even better, maybe they can celebrate with an Hispanic person and learn more about their culture.
Dia de Los Muertos is a traditional Hispanic holiday similar to Halloween and celebrated around the same time. I don't feel that non-Hispanics should celebrate this holiday unless they want to. I am a non-Hispanic and while I think Dia de Los Muertos is a very interesting holiday based on traditions, I don't necessarily want to celebrate it.