Virtually every time I've seen other dancers describe what Maddie was like, they've said she was nice and friendly towards them. And I'm including dancers from other studios in this.
I've only one or two exceptions to the above (really it came from a particular dancer's mother), and it started last season (Season 4.) Before then, even those two had agreed that Maddie was nice and supportive to everyone around her, and one of the sweetest girls the dancer had ever met... that no matter how much tension there was between Maddie and everyone else (likely alluding to complaints about favoritism and the like), Maddie was always there for them. I guess it must be obvious which girl and her mother I'm talking about here.
The other exception was what happened when Maddie and Gino were pushed into kissing on the show. IMO, Maddie should have handled that better, even with her being pushed by Abby and producers and whatever. OTOH, she was 11, and I think it's understandable that she wasn't keen on kissing boys yet (celebrity crush on Zac Efron or no), or knowledgeable about how to deal with this. Gino was embarrassed and said some regrettable things, and it was unfortunate situation on all sides. (You could see Maddie cringe going back over the situation in Girl Talk 2.)
I have also sometimes seen fans say that Maddie was more stand-offish than most of the other girls. They don't complain that Maddie's been mean about it; it's more that they sometimes say she hadn't been willing to sign autographs for random fans, when the other girls were.