They (well, Han) knew that the Falcon was trackable. If Han and Chewie could find it, then so could Kylo Ren/The First Order. (The only reason Han & Chewie couldn't find it before was because it was sitting in a junkyard, completely deactivated.) So it was just a matter of time before their enemies caught up to them; they needed to rely on speed, not secrecy.
They were going to Maz Kanata to ask a favor. It would have been ... counterproductive, not to mention impolite, to start out by attempting subterfuge, since Maz was famous for her ability to see right through people.
I suppose they could have attempted to disguise just BB-8 (Maz could hardly be offended by that), but keep in mind that the droid was part of what enabled the resistance spy at the castle to recognize them, and send the message that eventually saved their collective butts. Of course, it's also what allowed the First Order spy to recognize them, which led to their butts being in need of saving... but then again, we've already established that the Falcon would have sooner or later brought the stormtroopers on them.
(As far as why they didn't disguise just the droid, I don't think they realized how distinctive BB-8 was, nor how much detail the First Order knew about the droid. Also, how do you disguise a BB unit? Pretty much anything you apply to it is liable to "gum up the works", innit?)