The actual quote from the movie was "She can still outrun and outgun us".
The "still" could simply be a contextual qualifier. That is to say, even with the repairs Spock was able to oversee while Kirk and party were underground, the Enterprise and Reliant were both still severely damaged vessels, and apparently the Enterprise took the worst damage of the two. Spock's remark could mean that Reliant was not as crippled as Enterprise. This would say nothing about the military appraisals of their respective starship classes.
It was never clear to me what Reliant (and Saratoga after it) was supposed to be. A lighter cruiser than the Constitution-class ships? Some special type of limited-production Federation destroyer/warship? Or were certain ships like Reliant and Saratoga supposed to be mission-specific mods, fitted with special pods and other add-ons to fill specialized roles? (When the mission is over, they go back to starbase to be fitted for different missions)
There were other things seemed odd. The roll-bar thingy seemed very un-TREK like to me. The design seemed to be very 80s, with too much surface detailing that reminded me of STAR WARS and SPACE: 1999 and the like. But that's just me.