A/N: Just a few thoughts on what the in-universe reason may be on why Chell is a "mute lunatic".
Without A Word
GLaDOS was still talking to her, expecting her to respond to the implied insults and underlying threats in her words, but Chell didn't react. As the AI made yet another jibe about her test performance, Chell just portaled up to another platform to collect the cube resting on it, not even appearing to have heard. But that wasn't the case. It wasn't that she wasn't listening or was trying to tune out the computer's monologue - she was in fact paying very close attention to every word that was said - but she wasn't about to give GLaDOS the satisfaction of seeing her retaliate to the taunts.
GLaDOS wanted to see her suffer; see her break down and give up like the other test subjects that had snapped and starting scrawling half-insane ramblings about cake and companion cubes on the walls, but Chell wasn't going to let that happen. She knew that if she focused on getting through the tests, and didn't let herself panic even when faced with the dangers of acid pools and armed turrets, then she was going to stay sane and stay alive. But other than keeping calm, she had to also keep silent.
AI's were designed to learn based on the data that was inputted into them. They had perfect memories. Every sentence that Chell said out loud could be stored in their hard drives and analysed, used to seek out her weaknesses and turned against her in a hundred different ways. These tests had been strategically and sadistically designed based on what other test subjects had reacted to, what had strained and distressed them most, but if Chell didn't show any visible reaction, GLaDOS couldn't do the same to her. That was the reason she kept portaling the cameras of the walls where possible, depriving GLaDOS of the visuals of what she was doing as well as the sound. Don't speak. Don't react. Give nothing away.
By now, she knew nothing would be gained by attempting to talk or reason with the AI. Instead, the greatest power Chell held was in overcoming all the attempts to break her by calmly conquering each test, and taking from GLaDOS what she herself was denying. By listening carefully to everything the AI said - every deadpan remark and thinly veiled insult - Chell could figure out how her opponent's programmed mind worked, what GLaDOS wanted and what she was planning. That was what would give her the advantage. It was a few tests back that Chell had figured out the computer had sinister intentions - its promises of cake and jokes about pretending to kill her had hardly been convincing - but if she said anything now the computer would only learn and know how to better mislead her in future dangerous tasks.
And so the best thing she could do now was to simply concentrate on completing each test, infuriating GLaDOS in the process and using her reaction as a way to judge what may come next. Then she could figure out a way to get out of here, and she would do it all without saying a word.