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."

I agree with you. Even though Carly may not treat Freddie right at all times, Sam certainly does not as well. Carly uses his feeling to manipulate him (iLike Jake, iDon't Wanna Fight, iHate Sam's Boyfriend, iMake Sam Girlier), and Sam is full out abusive(pretty much every episode). Even Sam uses Freddie's feelings for Carly to her own advantage (iMight Switch Schools). Like you said, she got Freddie to help her sabotage the interview by saying that Carly would have a better chance of falling in love with him. Also, she sent Freddie a text message from Carly's phone, saying 'I love you.' Nobody is perfect, but this does not mean that Carly and/or Sam don't "deserve" Freddie.

Also, you're forgettting when Freddie tried to help Sam in iSaw him first because he didn't want Carly to be with another guy. (Did anyone else think that Shay looked cute? Just asking.) Granted, I am a Seddie person, but the example I put down shows that both Sam and Freddie can manipulate each other at times, so that makes them pretty even in my book. The fact that they use his love for Carly at times will be something to laugh at once they marry.