I can’t speak for the classic comics, but I have read the recent ones, and it varies a lot between them, both in art and story. The art is at least decent all around though.
The first series, Reclaiming Thundera, is a lot like the show; a series of short, episodic adventures. The next one, Return to Thundera, is where things start to get weird. Mumm-Ra ends up taking over, and things get darker, grittier, and implies some very child-unfriendly stuff.
The series after that, Dogs of War, is set after that. It keeps the tone of Return, but it gets a little stupid at times. It’s neat that there’s a race of dog people against the ThunderCats, but they go a little overboard when they mention whole planets of other human/animal hybrids. It also has some poor characterization that doesn’t really fit with what happened in the series before.
Hammer Hand’s Revenge is closer to the show and is set some time during it. It has the same tone and is a lot like watching the show, but I feel it gets weak towards the end and the finale was kind of a let down.
Enemy’s Pride is set just after the series, and it’s a lot like Hammer Hand’s revenge. It does a good job of building things up, but towards the end things start to get kind of stupid, and it’s as if the writer just stopped taking the story seriously.
I would recommend the Heroes and Villains/Villains and Heroes one shots. They do a nice job of explaining the back stories of many characters, and the stories are fairly entertaining.
I have not read the crossover comics yet, but when I do I’ll have to remember to post my thoughts. Overall, I recommend the one shots, Hammer Hand’s Revenge, Reclaiming Thundera, and Return to Thundera in that order.