As far as I know, the government doesn't define what "skilled" or "unskilled" labor is. Those terms are typically used by organizations which represent certain labor groups.
Sometimes, you may see a government agency or program that uses the term "skilled labor" but is aimed at assembly workers rather than airplane mechanics. There is an equal chance you may see the term pertaining to airplane mechanics and nobody else. It's just a descriptive term.
In Canada, one group (at least) identifes aircraft maintenance people as "skilled labor". Check out this linik for details:
Please do not consider the presence of this link as an endorsement.
Last but not least, trade associations are typically regional (state or provincial) organizations as are many employent and labor services. The regulation of Civil Aircraft Maintenance is administered by the national civil aeronatical authority of the country you are practicing in. Therefore, aircraft maintenance personell are often not represented in "skilled labor" demarcation lists.