DH always seems to worry a lot about our kids' popularity, and I'm much more sanguine about it.
A few years ago when loads of kids didn't come to DD's party he got in a big flap that we hadn't organised enough playdates, she wasn't well liked enough for people to bother etc (though it turned out it was just a bad date for people, other parties have been fine).
He always gets upset when he sees them with peers and they spend time on their own and only have one or two friends they interact with. Both kids have 3 or 4 good friends and other less close ones, and I honestly think that is fine. I would worry if they had no one or relied solely on one child, but they don't. I don't think even the 'popular' kids have more real friends than that, they just have more hangers on in a sense.
DH seems to feel very self-conscious about having not been a popular kid; I wasn't either, but I don't much care. I did have friends and they were good ones I never had rows with.
Our kids are nice, and aren't bullies or seriously bullied. I don't really see what we could do to make things otherwise. I'm sure his very socially pushy mum must have organised lots of playdates, and that didn't help him. I'm not about to tell DD to grow her hair and be into more of the same things as her peers so she's more 'popular'. That only makes one 'tolerated' and allowed to tag along as far as I can tell. Much better to give them confidence in being themselves and that they have the right friends.