I assume you are referring to the Mk3 Mondeo although this goes for previous models too.
Unreliable:
The Mondeo is a very reliable car when compared to similar priced rivals, e.g. Renault Laguna, Peugeot 406. People think they are unreliable because they have a mate whos dad had a clutch go on his 150k miles Mondeo. There are so many Mondeos on the road it's inevitable that you will hear a bad story about some.
Poor performance:
In what aspect? The range of engines is comprehensive and caters for almost everyone. Yes, the smaller engines maybe don't cut it with a car of it's size but every range of car model in this class has a weedy engine available. As far as handling goes, the Mondeo is regarded as the best handling car in its class, there's no arguing with that. For what the Mondeo is intented to do it is an excellent performer, especially when placed alongside it's rivals.
Poor Design:
The Mondeo wasn't intended to be a sports tourer, a fancy coupe or a supercar. It was designed to be a family car/repmobile and is designed very well to this specification. It's cheap to buy, practical, cheap to run, performs well and is reliable. It is perfect for your average sales rep with a wife and 2.4 children.
The thing that puts a lot of people off buying a Mondeo is the Ford badge, either because it lacks prestige or they have associated it with typical British engineering and build quality (i.e. Rover). I must admit though that when I was buying my Passat I also test drove a Mondeo TDCi. Compared to the Passat the brakes were poor to say the least and the interior build quality and solid feel wasn't up to the VW but by no means was it bad. I was in the position to be able to pay the extra £1k for the equivalent Passat but if I was a bit strapped for cash I would have settled for a Mondeo without hesitation.