Short answer: According to the definition of life, yes, Mitochondria are "dead".
To be considered alive an organism must meet the following criteria:
- organized structure performing a specific function
- an ability to sustain existence, e.g. by nourishment
- an ability to respond to stimuli or to its environment
- capable of adapting
- an ability to germinate or reproduce
Are Mitochondria organised?
Yes, they have a structure that allows them to metabolise energy brought to it by the cell.
Can Mitochondria sustain their own existence?
No, many of the genes that mitochondria need to function are no longer in the mitochondria. They need the host cell to provide much of what they need for them.
Note that I am pointing to the Mitochondria's genetics. They used to be included in the Mitochondria itself and have been moved into the cell host DNA. This is why I consider them to be "dead" because they are no longer their own organism, they are an organelle that helps the cell stay alive.
We can stop here since this disqualifies Mitochondria from being considered alive. However, for completion, I will continue.
Can Mitochondria respond to stimuli?
Yes, in order for it to divide it receives signalling from the cell. I would consider this a response to its environment.
Can Mitochondria reproduce?
Yes, their reproduction is much like a bacteria reproduces - through a process called fission.
Conclusion: Since Mitochondria cannot sustain existence, alone (thanks to no longer holding their full genome) they are dead.