Muslin is woven loose on purpose. Those gaps let air move through the fabric, so on its own a single muslin layer against bare skin keeps most babies cool enough. Add a singlet underneath and you close off some of that airflow right where it matters, at the chest and back. On a hot, humid day that can make a baby feel warmer, not cooler. We think about total layers for the day rather than adding a singlet by habit: how many layers should a baby wear in hot weather walks through that room by room. In most warm rooms, muslin alone does the job.
There are times a light singlet still makes sense. Air conditioning can drop a room well below outside temperature, and a baby who was fine in muslin alone at the beach might feel cold once you're back indoors under a vent. A fan-only room is different: moving air feels cooler on skin without actually lowering the room temperature, so muslin alone usually covers it. What should a baby wear in a room with only a fan covers that case directly. Right after a bath or once the evening cools off, a thin cotton singlet under a muslin layer is a reasonable extra.
If you do add a layer, keep it as light as the muslin itself, a thin cotton singlet rather than something heavier. A tight fit under a muslin piece traps heat instead of letting it move, so check the fit the same way you would the outer piece; how should muslin baby clothes fit covers sizing and room to move. Wash both layers the same way: cold on a gentle cycle, tumble dry low or hang, and skip the fabric softener so the muslin's weave stays open.