Is linear growth in a startup good or bad?

It depends.

If you're a bootstrapped business, and your goal is to have the company provide a living income for the owners and staff, it can be fine.

If you're VC-funded, linear growth is often considered by the investors to be insufficient to generate the return on their investment that they consider worthwhile continuing to fund.

What does 'growth' mean... is it making a profit? That's a business then.

Number of users going up linearly and making a loss? That's a bankruptcy.