As someone who campaigned in multiple states for Sanders and was active in the online community, you're full of it. TONS of Sanders supporters are pissed about this.
As far as why some people think she comes off as mocking? She campaigned on the idea that Sanders voters were middle class straight white college kids, the famous Bernie Bros, like the Obama Boys of 2008 before them, and then she makes a comment about people living in their basement.
That's why the hashtag was #basementdwellers even though she didn't say that. Because that is a very negative stereotype, perhaps topped only by neckbeard, that 100% coincides with how she portrayed Sanders supporters in the primary. So that's what people thought of when they saw her use the word basement.
You'd think as the super prepared detail oriented person she portrays herself as, and with her massive staff, she might have had a clue. And remember this was from February, so it was right in the middle of the whole Bernie Bros shtick. It's not hard to see why people took it in a negative fashion.
Of course the real thing to be pissed about was when she said she occupies the center left to center right. Reminds me of that old interview where she asked if it was possible to have the mind of a conservative but the heart of a liberal. The answer is yes. But we call those people Republicans. I would never support someone who referred to themselves as representing the center right. Fuck the center right. The center means pro-corporate. I don't care if 1% of black and gay and female people get to join the 1% that exploits the other 99% economically. And that's the only thing Clinton and Obama ever offered. I'm not voting Trump either of course. But Hillary's failure with millennials is on her. She doesn't support the Sanders agenda. Notice how he repeatedly brings up how bad Trump is. Because he can't make a positive case for her. He did the same thing traipsing arounds Vermont for Fritz Mondale in 84 saying Reagan was 100 times worse. Poor guy. 30 years and in the end he accomplished nothing. At least he tried unlike the Clintonian 3rd Way Dems.