Yes, that looks like a piece of lead tape. It is very common now in badminton rackets, usually somewhere under the grip. The purpose is so that those rackets will pass QC inspection before being sent out to buyers. The less lead 'balancing' tapes a racket has, the closer it is to the theoritical specs by the designer.
Don't be too concerned about that, even it slightly reduces the power of the head/frame. As long as u enjoy yr rackets just play and practice your shots. Also, that piece of lead tape weight is rather negligible for a tennis racket unless it is an ultra-light one.