In my experience, and please remember, this is in my experience:
I have straddled both sides of this fence through my life. My parents where "laborers" (my dad build boats & my mother was a cook at a nursing home). I also was lucky enough that my father was also somewhat intellectual and read a lot (not something I have found, again in my experience to be as common among the groups of people I was exposed to that you would typically associate with trades & manufacturing/construction laborers) so between that and being in the Army (which I also observed as one of the best upward mobility machines in terms of class, pay, benefits and achievement, ironically) he knew lots of people who were Engineers, Architects, Business owners etc. as well

From all that experience, as well as my own personal experience having friends from multiple types of backgrounds, I have observed the follow again, my own experience and opinions:

This is what I found to be true:
- People who do trade jobs are respected by most (if not all) my quote unquote 'intellectual' or 'upper class' friends (those smart enough to value their labor, myself included) Even my friends or the friends of my parents growing up who where multimillionaires (including one family who had 2 state governors in it). I know this isn't universal, just a general observation.
- They have always tended to favor policies (politically speaking) & outlooks that would actually benefit this group of folks the most
- There isn't any classicism bias in as so far as I could tell among anyone I know who would be defined as well educated, wealthy, upper class etc.

However, I see this stigma come up a lot, and I have to wonder, is it those looking down or those looking above who perpetuate it? I say that because:
- Being around a lot of families that had roots in working trades, I found they often would have resent against those who they considered 'upper class', without much or any evidence as to why this opinion has formed. I always found this fascinating, because its typically their target market (with exceptions of course).
- Most trades people I knew typically didn't seem to value education as much (i know its a stereotype, but it did play out in my experience)
- Many, many MANY (with the exception of the ever dwindling Unionized tradesmen) favored economic/political policies and had outlooks that would be long term damaging to them (One of the biggest protesters where I grew up of universal health care? Ironically, those who aren't poor enough for govt assistance but aren't well off enough to pay full flight for their medical care (even after insurance). E.g. Those who would benefit the most!)
- And often, these are the folks who look down on their own jobs the most.

Now, all of these have caveats, exceptions, edge cases, what have you. I knew those who worked trades that did value education, had lots of logical & grounded philosophical beliefs, etc etc etc and I grew up around people who had carpentry businesses who were multimillionaires before they were 40. So its not hard and fast how this plays out.

I know i'm rambling a bit here, but I feel like this stigma may not be caused by those looking down, but perhaps, those looking up.