I wouldn't go so far as to say that multiplayer is easier, but it's true that if you're able to win routinely on emperor you're likely above and beyond the average public games hosted online. What you first need to remember is single player and multiplayer often involve different tactics. In multiplayer you have to keep a good balance of military strength and technology, if you fall behind on either you're likely going to be crushed. While I don't doubt that players tackling emperor and higher as their preference would make very competent multiplayer contenders, they'd likely have to spend several games adjusting to the difference before they were truely "good". I'd also think it would be easier to adjust from single player to mutliplayer, since the other players capturing your cities should give you an idea of what you're doing wrong.
As far as my personal experiences, when I started playing CIV me and a friend used to direct ip and play big games with a lot of ai. We would try to have as little interaction as possible, we weren't allies but we generally wouldn't fight until the game was over. We started on noble, and then prince and eventually found ourselves quite comfortable on monarch for awhile. Thats when I started playing online, and although I was reluctant and doubted a game like civ would be practical with 5+ random people playing, turn timers and quick speed really save the day.
Fast forwarding from then to now, public gamespy games don't really give me much of a challenge. I can join a public ffa on gamespy and chances are I'm going to win, and the times I do lose, it's often a couple players attacking at once or getting dragged down in a long stagnant war that doesn't really end. Most players in public games don't tech very well, often don't have a good understanding of slavery and their economy just doesn't make the long-haul in general . Although they are more level-headed military commanders then the AI(ussually >_>) and harder to take over when you're close in power(since war in civ isn't really rocket science).
As far as single player I'm stuck in the zone where monarch is way too easy, and I basically can't lose, while emperor is ussually quite difficult. That said, I generally play the same style for single player as I do online, quick speed and I never slack on military, don't really focus on wonders and just try to win straight up making the best of my land. The wins I do have on emperor are when I decided to really focus on a specific strategy for beating the ai, rather than just trying to keep my civ as 'strong as possible'. Also from what I've seen from others wins on immortal and deity are all about the liberalism race, out-teching the ai, and/or unrealistic axe landgrabs that simply wouldnt work on a somewhat-competent human. You only get certain specific opportunities to pass/catch the AI on these difficulties, and if you don't then you lose.
As far as the ladder, I wouldn't worry about being able to tackle emperor first, as I said before both require different strategies. If you're finding the average public games too easy you should find the ladder games competitive yet enjoyable. When I was playing ladder in warlords I sure as hell couldn't scrounge a win on emperor yet i was still holding my own and had a win ratio over 50%. I played with many of the top ranked players, and even have wins against some of them. Unfortunately for me cton doesn't seem like a popular stlye of play on the ladder anymore, and i'm not a big fan of teamers. =(
To sum it all up, while good playing habit, and the general thinking and management of your civ are the same for both multiplayer and singleplayer, the strategy isn't. And although this post is long, there is still a lot more I could type about how different I feel SP/MP are from eachother. I wouldn't so much worry about which you find more difficult, as I would which you enjoy more.
However, this is all just based on my personal experience and others may vary.
Edit: Oh yeah, i almost completely didn't mention turn-timers. Some people may not be able/comfortable playing with the blazing turn timer. You really have to play and think fast. The ability to think on your feet, and quick management skills are required or your turns may feel far from optomised. There are often times when I'm struggling to get everything done within the time limit(and some where I don't) and I've played my fair share of online CIV.
some players online are very very good, but the large majority just are .. Meh.. a guy in my last game was playing at chieftain, he started talking about Settlers/Workers popping from huts and I was like..... they do that.... lol
