That's a good option when replacing the internal battery isn't practical, just be sure to buy an external battery that's not too bulky but still can provide enough power for a tablet. It's not going to 'damage' your tablet, but it's still a bit of a kluge.
But how old is your ONN tablet? Instead of spending money on extending its usage life, would putting that money towards buying a new tablet be more economical in the long-term?