Is it ok for me to let our baby sleep on the couch?

No. It is never safe to let your baby sleep unattended on the couch. The baby may roll off unexpectedly, and the couch is usually soft which is a risk factor for sids (if baby turns prone). In addition, baby should never sleep on the couch with an adult, because this has been shown to be a significant risk factor for sids. The couch, while comfortable for an adult nap, is not a safe place for baby.
No. Be careful that your baby doesn't roll off the couch, there are no side rails.
No. Young babies should not sleep on a couch, nor on an adult bed. Neither are as safe as a baby bassinet or baby crib. If sleeping on a couch, a baby can fall off the couch, can get stuck between the couch cushions, or have some other unforeseen bad thing happen.