Only For Ramadan?
There are some which claim that this dua should be recited everyday of Ramadan. However, there is no support for these claims. From the hadith only specific day mentioned to recite this dua is for laylatul qadr.
This does not mean it is restricted to this one day. It is a dua which can be recited whenever one wishes to ask for forgiveness from their Lord.
The addition of Kareem, correct or incorrect?
In Jami At-Tirmidhi 3513 there is some confusion as this hadith adds a variation to the dua with the word Kareem or Karim, which means the most kind or generous.
اللَّهُمَّ إِنَّكَ عَفُوٌّ كَرِيمٌ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي
Transliteration: allahumma innaka afuwwun kareem tuhibbul afwa fa’fu anni
Meaning: O Allah, indeed You are Pardoning, [Generous] You love pardon, so pardon me.
This slight variation is not the authentic sunnah of Prophet Muhammad (ﷺ). This does not make this dua wrong, Allah loves any act of worship and the niyyah or intent behind this dua is still pure.
The difference, however, comes in rewards. The dua which adds the variation Kareem which is likely a mistake from a scribe would be rewarded. Again Allah loves the believer who is humble and shows humility to ask for his forgiveness. But on a scale of one to ten, this would be a level one dua.
Reciting a dua in tawaf, before or after sadaqah, in difficulty, or during Ramadan in last ten nights increases the rank of a dua.
Similarly, reading the dua in Arabic or sticking with exact dua from Quran or keeping to authentic Prophet Muhammad narration would be better.
For that reason it is better to stick with the original version “Allahumma innaka afuwwun tuhibbul afwa fa’fu anni”.