Alot of people see it (especially Creddiers, because it's our ship and obviously Carly's reactions are part of that), but whilst it is somewhat of a negative trait, how you percieve it most likely comes down to which ship you are on.
Seddiers see it as Carly being manipulative, "evil", taking advantage of his feelings.
Creddiers see it as Carly being insecure, not wanting to completely shut down the idea because she knows both options could be bad.
Freddie also isn't immune to "blame". He's taken Carly's side occasionally simply because he "loves her", and Freddie doesn't have to do anything, no matter who asks him to. He's been shown to help or hinder in situations because he likes/loves Carly, for example when he helped Sam sabotage Carly's private school interview so he wouldn't lose her.
Is that the type of person who 'deserves' Carly?
Also, do not forget that SAM herself has used his emotions against him, in that very same episode, Sam was the one who told Freddie to help her so Carly would stay and have more of a chance to fall in love with him.
Is that the type of person who 'deserves' Freddie?
Then the other thing, is Seddiers claiming that if she is jealous, and only starts to like Freddie after someone else likes him, that's it would be somehow "bad" because she's had the chance to date him before.
Again, Creddiers would say it's that jealously forcing Carly to realise her true feelings, or to stop supressing them. It's not "bad", it's just the situation, a "you don't know what you've got until it's gone" thing.
Now I don't want to get into "Sam vs Carly", but Sam isn't free of complaint in how she treats Freddie either, and just as someone might say "Carly doesn't 'deserve' Freddie because she manipulates him, and leads him on, not being honest enough to tell Freddie once and for all to stop it," someone could go "Sam doesn't 'deserve' Freddie because she's physically and emotionally abusive, and could be hiding her true feelings instead of being honest with Freddie."