The importance of these mottos comes with the Party's ability to control. If someone can convince you that the sky is not blue even though through your two 20/20 eyes you can look at the great expanse above you and see that it is blue, they have true power over you. These are the bigger ideas dealt with in 1984: war, peace, slavery, freedom, ignorance, and strength. To be able to convince the proles or the people that freedom is bad or war is actually good is quite the feat. This question seems to answer itself as you use the word paradoxical and then give examples, so it is important for you to analyze each one. Is ignorance ever strength? If so, how?
Distorted truths, outright lies, the degradation of language and the widespread use of propaganda ("newspeak") are important ways that the Party tricks people into beliefs that are not true and ultimately controls their minds. These mottos are evidence of that because each word in the word pairs is an antithesis of the other. "War" is the opposite of "peace"; "freedom" is the opposite of "slavery" and "ignorance" is the "opposite" of "strength". By saying "war is peace", the mottos are contradicting themselves and they make no sense, unless you are a twisted member of Oceania's Party. If "peace" is something to be desired and "war" and "peace" are the same (war is peace), then it would be just as desirable to have peace as it would be to have war. This makes no sense. Same with the other mottos. No one wants to be a slave, but if the Party can convince you that slavery is the same as freedom then, hey, no big deal. If ignorance means one is strong, then everyone would seek after ignorance. See how absurd this is? That's the point of the irony.