Three review tiers on Chinese e-commerce:
好评 (hǎopíng, positive),
中评 (zhōngpíng, neutral),
差评 (chàpíng, negative).
刷 (shuā) here means 'to fake / artificially inflate'. Compare
刷单 (shuādān, fake-order fraud),
刷量 (shuāliàng, fake metrics),
刷粉 (shuāfěn, buy fake followers).