I don't judge by which part of the plane people are sitting... or how they got their tickets (paid themselves, paid by miles, paid by company, gift from a friend (or special friend), whatever) - just behave? Certain airlines I won't fly because I find the overall mood and attitude of passengers to be awful... and it's probably of no coincidence that behaviour correlates with low cost carriers. Some experiences have been better than others (JetBlue I don't think is a true LCC but is sold as one and I adore them - but Spirit, Ryanair, EasyJet, Spirit, Frontier - all can just... ugh, nope, never or never again. Southwest is acceptable. Haven't had the opportunity to try Norwegian yet).
As far as legacy carriers though, I think the more an airline makes a visible difference between classes... the more passengers will act like there is a difference and thus behave and think differently. And passenger demographics play a huge role in that as well. I fly between Scandinavia, England, and the US regularly and the difference is stark. People really do live up to stereotypes sometimes... I'm actually a big fan of free-for-all boarding (minus assistance for those who've made legit prior arrangements for pre-boarding ahead of time due to disability - they do not mess around with that over here; they go first before boarding of any kind is announced and it's quietly done without a fuss). The absurd number of boarding groups and hierarchies is... well, absurd. That sort of thing definitely encourages a "well first class people gotta be first, must think they're special and better than everyone else" sort of thing. I like the dedicated boarding lane to board whenever for those with whatever elite status/cabin and one dedicated for everyone else - two groups, done... rather than 20 different boarding groups based on ticket class and elite status. Also, having economy class passengers walk through first/business class? Oi. It's a logistical issue for one, but also, and while I get it's like free advertising to show off the product or whatever, it's also sort of rubbing it in their faces to see what they're missing when they get to the back and are squished together and pay $12 for a pile of mush between two soggy pieces of bread-like substance at some point. Generally, when I'm sitting up front, I feel bad for the people in the back. They have my sympathy and I'm one of them pretty often.
As for the reverse - those in the back mocking those in the front as if they think they are all high and mighty, I do encounter a fair amount of that. I fly up front and in back, it's hit or miss, because I'm paying for all of my flights myself. But really, I judge people on their behaviour regardless where they are in the plane - cabin or staff. I'm judgey about behaviour.