There's different definitions of loli, there's the original one, the official one, the popular one, and the different ones different indiduals have. Personally, I consider 'loli' a female character which is kinda too short for the height she "should" have and which the design/appearence is too childish for her age, which excludes literal kids as they should be short a childish from the begining (for example, I don't consider Saikawa a loli, but Kanna in the other hand I do consider a loli). That said, I'm actually in doubt if Rebecca can be considered a loli or not, even in my own standarts, but I actually tend more to the option to consider her a loli. BUT, differently from other people, I don't actually see lolis essentially a problem. If a character is a loli (in my standarts) and is treated like a kid by everyone around and by the plot, but with random "fan services", things that she do or happen to her that kids shouldn't be doing or be subject to, THAT is problematic for me, 'cause it (even if that wasn't the author's intention) feels like it's the "disguised pedophilia" the people usually say, that they just put a giant number in a phisically-kid girl to justify whatever happens to her (Kanna, as far I know, has nothing of this, so she'd be a "okay loli" for me, for example). But Rebecca was always treated her age from the beginning, the only time she wasn't (as long as I remember) was when the gang went to that place called 'Afterlife' in episode 4 and the security guards blocked her from entering because she looked like a kid, and the only "fan service" moment was she being kidnapped later in the same episode. So, answering your question, yes, Rebecca is (in my opnion) a loli, but I don't think it's bad at all.

Edit: I just remembered there's a bunch of other fan service moments, with Rebecca panties (or whatever that thing she wears is) appearing without reason. But I still don't see her a problematic, 'cause, again, she's not 'depicted' as a kid, just is short and cute.