Is it more difficult to mount non-tubeless tires on tubeless-ready rims than non-tubeless-ready rims?

Typically yes, but I am sure there are edge cases (read poor manufacturing tolerances) where this isn’t the case.

In order for tubeless tire setup to work you need tight tolerances as the bottom of the tire bead needs to be in firm contact with the rim bead seat in order to make a firm seal. As such the bead seat diameter (BSD) should have less variability and be closer to specifications.

Regular non-tubeless rims are not under such tight constraints. Manufacturers often undersize the rim (or have less concern about batches that are slightly undersized). Depending on who you believe this is done in to make mounting non-tubeless tires easier or to ensure good batch yields - no one will want to release a rim that is out of tolerance and too big as no one will be able to use it and it would become a warranty nightmare.

Of course this isn’t a complete universality, as there have always been tight tire rim combinations (when tolerances collide) long before tubeless, but with the tolerance improvements we are now consistently closer to the true BSD specification making tire fitting a little harder.

There is always the potential for tolerances to make a particular tire, rim, or combination hard to mount.

A rim being tubeless or not per se does not dictate how hard it is to mount a tire on. Tubeless rims aren't made tighter, i.e. their target diameter for the bead is not a larger number than non-tubeless.

There has always been a cat and mouse game in cycling where no manufacturer of tires or rims wants their parts to be subject to tires blowing off. So sometimes they make decisions that nudge things in the tighter direction; ie smaller bead seat diameter for tires and bigger for rims, and truly difficult mounting situations can result. You can find some tubeless parts that show this phenomenon, but it's not categorical to tubeless. It's probably less common if anything if I had to guess.

In practice, almost all problems people have mounting tires on to tubeless rims relate to them not being deliberate enough in getting the beads sunk in the middle well of the rim. Doing this all the way around is how you make the final tight spot easy. This has to be done on purpose with tubeless rims in a way it didn't so much before.

If you gathered data you might find that tubeless rims had a larger BSD on average and are harder to mount by that definition. Or, you might find that the tighter tolerances are working in both directions, and average BSD is the same. That data doesn't exist. Another definition of which is harder to mount is for which type is it more common to encounter a rim with a BSD that's simply too large and has to be brutally fought to get a tire onto. In my experience that would be non-tubeless, presuming one has good technique with tubeless center wells, as above. I very rarely have that struggle with tubeless rims.

You'll read that Conti road tires err tight and there may be some truth in that, but only to a marginal effect compared to the difficulties you'll encounter if you don't do the above.

@NathanKnudson already mentioned the well in the middle of tubeless rims. This merely illustrates why it's necessary to use that well. The picture below is from Slowtwitch.com.

For readers not used to tubeless rims, the well or center channel has a narrower diameter than the rim's bead shelves, where the tire beads sit when inflated.

I'm not sure what the optimal sequence of events is. However, when I mount the first side of a new tire, I get that side's bead in the well. Then I work the other bead across the rim's sidewall. If I get stuck, I go around the rest of the tire and make sure both beads are pushed into the well, then I return to the bit of bead that I couldn't move earlier.

I don't follow rim standards, but I suspect the bead shelves have a larger diameter than the equivalent but of a traditional, non-tubeless rim by design.