if you're shy, you must have an extroverted co-founder. i always find meeting founding teams where all founders are shy an especially underwhelming experience.
I'm not sure what shyness has to do with the founders' product. If you're basing you opinion of the product on the level of extroversion of its creators, can I suggest that you're looking for the wrong thing?
At some point you need to "sell" your product, even if it's only to the sales team you're appointing or to a VC. If you can't then I warrant you'll not get the best deal for yourself and probably not enthuse others to help you produce the best product (for some value of best!) that you could.