For the time being, Unikitty! is actually an okay show for Cartoon Network. It's not bad, not good, just okay. The series, to my knowledge, has no relations with The Lego Movie despite the titular Unikitty being a prominent character in that movie. Here, the show is two dimensionally animated unlike the Lego Movie. As such, it's almost easy to forget that the characters are supposed to be Lego pieces. However, I can see some reasons behind this change namely that CGI would've been more expensive, and the characters are more visually expressive.
As for the characters, they're fine, albeit one note. Unikitty herself is a mixed bag for me. Imagine that you had Pinkie Pie from Friendship is Magic, and Star Butterfly from Star vs. the Forces of Evil, and you blend them together, and you'd get Unikitty. Princess Unikitty is always upbeat with trying to find the best in every situation, but she also has the tendency to get enraged. You wouldn't like her when she's angry. Now, there are some things about Princess Unikitty that are good, but I can also see how she would be easily annoying for anyone to genuinely like. Not featured in the movie, there's Puppycorn, her younger brother (even though Unikitty's a cat-unicorn-hybrid), Dr. Fox a scientist, Hawkodile the hawk-crocodile bodyguard of Unikitty, and Richard a grey Lego block who functions as Unikitty's royal advisor. Oh, there is also Master Frown the main antagonist of the show who serves as an antithesis of sorts to Unikitty in which he revels in the suffering of others. Other than that, he is a one note bad guy.
The show itself is fast paced much like the movie, which can be a bit of an issue because it seems that the show never shuts up and gives the audience the time to rest and digest everything going on. The animation is also very fluid, which helps with the upbeat tone of the show. I may watch the show every now and then, because there really isn't anything deep about the show other than generic morals here and there. You really wouldn't miss much if you missed an episode. It acknowledges that it's stupid, so it doesn't have the need to feel that it should be anything other than a lighthearted comedy. As I only watched small parts of The Lego Movie, I can't definitively say you'd love it if you were a fan of the movie, but I would suggest giving it a try and if you don't like it, you could drop it and maybe watch the other Lego cartoon series.