I would try to start slowly. Lecturing your friends or confronting them for a problem that doesn't really involve you isn't going to look good for you, and could very well make Mike feel very uncomfortable.
Think about it this way. Mike probably doesn't feel comfortable enough to confront Bob on his own, so imagine what he'll feel if someone else does it for him.
On the other hand, maybe Mike actually doesn't mind cleaning up, in which case you'll kind of look like a jerk for confronting Bob.
So let's look at some indirect strategies that can work without escalation.
Help Mike, Invite Others
Stand up and say,
Hey guys I'm going to clean up our mess a little to thank Mike for always having us over. Least we can do is not make him clean up after us. Bob, want to help me?
You can include other names, too, not just Bob. However, be sure to specifically say the names of the people you are inviting to help. Otherwise, you will create the Bystander Effect.
If you ask them specifically for help, then they will have to publicly say they are not willing to help clean up their own mess.
Help Mike Drop Hints
If Mike drops a joke, add in. Increase the pressure. Mike drops a comment on Bob's mess, maybe you say something like:
Yeah Bob, what's it like having your personal own maid?
Be careful. You mentioned that you and Bob aren't as close as some of the group dynamics. Only do this if Bob can take a joke from you.
You may not be trying to escalate things, but if you piss Bob off, he might escalate them for you.
Talk to Mike
When you guys are hanging out and it's just you two, mention how you've noticed Bob doesn't help.
Ask Mike if it bothers him. If you can get Mike to stand up for himself, it will be much more effective. The problem really is between Mike and Bob, making a lot harder for you to help without looking like a jerk.
And as an absolute last resort, which you should at least clear with Mike first........
Talk to Bob
This is really a difficult option. Again, this isn't your problem, so going to Bob and pointing out his offenses to another person isn't going to look well on you.
So here's what I would do. Shift the focus away from Bob's behavior. Completely.
If you talk about Bob's behavior, AT ALL, he WILL get defensive. When people get defensive, they stop being reasonable. So talk to him like you're on his side.
Say something like:
Hey Bob, I noticed lately that Mike has to clean up a lot when we go over to his place to hang out. I'm going to start helping him so that he doesn't have to clean up after us. If you could help out, it would make things a lot easier and really help out Mike. He's been a good friend to us.