We would not recommend it. Cosmos do not ask for much: they thrive in fairly poor soil, just make sure the soil drains well (and the plants are not in puddles). Too-rich soil may result in thin-stemmed, weak plants; cutting them will only hasten their demise. Stake tall stems again heavy rain or wind, when forecast. Or plant the flowers closer together than seed packages or plant labels advise so they can support each other. Pinching off flowers encourages blooms. Pinching stem tips can reduce height and encourage branches to grow but it is not necessary.
We would not recommend it. Cosmos do not ask for much: they thrive in fairly poor soil, just make sure the soil drains well (and the plants are not in puddles). Too-rich soil may result in thin-stemmed, weak plants; cutting them will only hasten their demise. Stake tall stems again heavy rain or wind, when forecast. Or plant the flowers closer together than seed packages or plant labels advise so they can support each other. Pinching off flowers encourages blooms. Pinching stem tips can reduce height and encourage branches to grow but it is not necessary.