How true is it? Do interviewers really look down upon Python?

Some do. Some don't.

Here's the best advice I can give you on matters like these (and on similar matters, like choice of dress): prepare yourself and act in accordance with the job YOU want. Remember that you are also interviewing them, so trying to appease every interviewer's arbitrary whims and expectations may well lead to your own expectations/needs/desires being unmet. Pay attention to what they ask you, how they work with you, and what they expect you to do in the interview setting, because that will give you a glimpse into what it will be like to work with them.

Here's what I mean: let's say interviewer A insists you use Java and constantly picks on minor syntax mistakes in your code. Guess what interviewer A is most likely going to do every day on the job? That's right--extol the virtues of Java and pick apart your code reviews, and pressure you and everybody else to do the same. Maybe you like Java and are a stickler for some syntactic style, and maybe that could be a good thing for you--but maybe it isn't.

Let's say interviewer B doesn't care what language you use, or even tells you to use pseudocode if you want. Chances are, interviewer B will be just as flexible and freewheeling on the job. Maybe you like flexibility, and maybe that could be a good thing for you--but maybe it isn't.

Every interviewer is going to have some style they follow, which is a reflection of how they work in general, and you'll probably be happiest when working with those who share your own style. The best way to find them is to self-select by choosing the languages/approaches/problem solving styles/whatever that you like. Getting "rejected" by people whose working styles clash with your own really isn't a bad thing at all (and trust me--if you end up working with them, you'll see what I mean).

One thing to note: large companies are large, so there's a good chance you'll end up encountering a cross-section of styles and expectations in a single interview loop (because some of them will source interviewers from different corners of the company). Even so, I strongly recommend sticking to your style rather than trying to appease every interviewer, both because you'll be more comfortable and perform better, and because even large companies tend to have overall trends (as an example, Google has a fairly conservative/pedantic engineering culture with a lot of people like interviewer A above; Facebook tends to have more interviewer B types), and you still want to end up somewhere that fits with what you like.

Oh, and because it'll make for an entertaining interview debrief for people like me who have to sit in on them...

Bottom line: if you like Python--for whatever reason--then you should use it.