Yes and No.
So - On the Yes side - a Giant Crab (able to be ridden by a Human and let's put aside the exoskeleton weight limit thing for a moment) is going to be big (duh!), is going to be armored and has 2 offensive weapons.
This would mean that in the role of say Heavy Cavalry, they would fair pretty well - able to smash apart an enemy line, the terror factor of a giant pincer and being able to hoist a man up and cut them in half is not to be underestimated in terms of psychological impact.
On the No side - What made Cavalry such an effective tool is speed a Galloping horse is faster than a Human. Even a nice steady canter is much faster than a mans Jog - and this is where Cavalry came into it's own - the ability to outflank and outmaneuver infantry. This also includes your scout cavalry - being able to go a great distance at speed and report what you saw and come back.
Then you've also got the harassments of your enemies supply lines - There's some great examples of this in the Sharpe TV series that shows how that works - suddenly a light cavalry division comes around the corner, engages a slow moving supply train slash, Slash SLASH! then disengages before they can rally to a defense and return fire.
In this area, even a Giant Crab, assuming the same speed of the fastest crab is only capable of 3-4 m/s, compares to a horses' 14 m/s.
So this form of Cavalry a Crab wouldn't be effective.