This is not a ripoff of Dexter's Lab, but it's more similar to Fairly OddParents, but you could say Fairly OddParents is a rip-off of this show, since Butch was inspired by this past pitching to Nickelodeon. But these 2 shows have a lot more different things, so neither of them are rip-offs of one another. Let's start with the similarities
1. The general formula of both shows is the same. Protagonist has a conflict, so they have to ask a pair of people that can do almost anything to help them. Their help turns out to have a big disadvantage though, making them have a bigger problem than before, so they need to get rid of it, but it's hard to do so, so they need to do something to get rid of that thing. Episodes end with them learning their lesson.
2. Some episode plots in Johnny Test are similar to Fairly OddParents episodes. Johnny in Black and White = Tiny Timmy, Johnny Fu = Kung Timmy, and 101 Johnnies ≅ Too Many Timmys (the congruent sign means that one episode is actually a rip-off of another)
As for the show itself, its first 3 seasons are really good, especially Season 3, with episodes like Johnny X and the Attack of the Snowmen, Johnny Applesauce, Johnny'mon, Bathtime for Johnny, Johnny Holiday, When Johnny Comes Marching Home, Johnny Test in Outer Space, Johnny Cart Racing, Return of Johnny'mon, Johnny Dukey Doo, and the two part season finale, Johnny X: A New Beginning and The Final Ending.
Season 1 is no slouch though, with the series premiere being the best series premiere out there, along with Chowder's Froggy Apple Crumple Thumpkin. It also has Johnny Test: Extreme Crime Stopper, Johnny vs the Brain Freezer, and Johnny Dodgeball. The Johnny X episodes aren't as good as the ones from Seasons 2 and 3, though.
Season 2 while not as good as Season 1 and the worst out of these seasons is still pretty good, with 2 pairings being great. Revenge of Johnny X, The Enchanted Land of Johnnia, Johnny X Strikes Back, and Johnny vs Super Soaking Cyborgs.
The reason this show is great is because of the plots of the episodes and how they get solved. Some episodes are heartwarming, like Bathtime for Johnny. It also has great action too. This is the best 2000s show along with the first 5 seasons of Fairly OddParents (the funniest show ever).