Brand extension can be good or bad for a brand depending on how it is being implemented. Knowing your customer segment becomes very important while brand extension.
If your brand extension meets the customer requirements and sends across a consistent message as per your brand identity, it is good for your brand.
But if a brand does extension in a totally different product category, leaving the consumers confused as to where the brand is heading and what is its future, then it can lead to brand dilution which is bad for the brand.