What’s wrong with my tomato plant?

New to gardening. The plant pictured is my first tomato plant that just randomly popped up in the garden, so thought I'd give it a chance.

Compared to the others I'm growing, its leaves are smaller, textured and somewhat brittle. Older growth seems to keep dying as the plant grows. It's produced a few flowers, but some have already dropped off. Other plants in same soil and same location seem to be thriving (though they were seedlings bought from the garden centre).

Is it likely disease or environmental? Could it be the variety?

Thanks!