On one hand, Mathieu is an electric playmaker and a high-energy difference-maker. On the other hand, he can be targeted by bigger receivers and is overaggressive.
There is no doubt that Mathieu was the biggest difference-maker among cornerbacks in 2011, but he was just as certainly not the best cover man. He's aggressive to a fault and is beaten by smart receivers and quarterbacks on a regular basis.
Rob Rang of CBS Sports validates this line of thinking:
Is a tenacious defender with strong, active hands to rip the ball away. Excellent ball skills. Minimizes his natural height disadvantage by timing his leap well in jump-ball situations and competing throughout the catch process, ripping away at the ball as he and the intended receiver are descending... Possesses the suddenness to make up for a miss-step but does not have the elite straight-line speed to recover against a well-executed double-move and accurate pass. Trusts his instincts too much and can put his teammates in difficult positions by drifting to where he anticipates the quarterback will be going with the football.
Nolan Nawrocki of Pro Football Weekly summarizes Mathieu well:
A high-risk, high-reward wild card, Mathieu is a talented, cocksure slot defender and instinctive ballhawk capable of infusing a secondary with swagger and playmaking ability.