Is it okay if I do my homework on the bus, laying on my bed, on the couch, or in other similarly less-than-stable positions?
Probably not a good idea. Anything that keeps you from being in a focused, studious mindset should be avoided. This includes unstable drawing positions, busy/crowded/noisy spaces, messy environments, drawing on lined or graph paper, and so on. You absolutely want to put your best effort into completing the exercises, as that is the way you're going to get the most out of them. Being in a less than optimal mindset will cause you to work sloppier, which is something you want to avoid at all costs. If you don't have any kind of a reasonable table or desk at home, consider going out to a public library instead.
Keep in mind that we want to be as intentional and meticulous in applying all of the methodologies and strategies we introduce throughout the course, and it will be very tempting to rely more and more on your instincts or auto-pilot as you progress through the course, and the kind of situations that leave us less focused and less stable will lean into that. The goal for what we're doing in this course is essentially training that auto-pilot to make it more reliable, so that when you draw outside of this course, you can rely on it to do all of the heavy lifting, freeing your brain up for the creative decisions of composition, design, and narrative which actually make your work special. But to get to that point, we have to make sure that as we do the work assigned here, we're giving it our all.