Some feel that having a mediocre co-founder is better than having no co-founder at all, which I wouldn't agree with at all.
Let me say that I think that having a co-founder is great, so if you can find someone who:
1) Has passion for the idea and concept and is willing to share in the risk.
2) Is a hard worker and someone who can bring talent to the table.
3) Is someone you get along with but can have fruitful discussions and respectful disagreements with.
Then I would encourage you to bring that person on as a co-founder. But I would not settle for anyone who lacked any of the above traits because you're worried about not having a co-founder.
So, in summary, co-founders are great if you can find one, but certainly not essential to startup success.