He's blaming himself. There's really no other way to understand "awful" and "terrible" here other than in the sense of denigrating his own behaviour. This is confirmed in the next line beyond your quote:
So when I told old Spencer I had to go to the gym and get my equipment and stuff, that was a sheer lie. I don't even keep my goddam equipment in the gym.
There'd be no need for him to describe his gym kit as "goddam" equipment unless he wanted to emphasise the ridiculousness of his lie. At the end of the preceding chapter, we see that he's concocted his lie because he doesn't like having to talk to Spencer and he wants to get away:
I felt sorry as hell for him, all of a sudden. But I just couldn't hang around there any longer, the way we were on opposite sides of the pole, and the way he kept missing the bed whenever he chucked something at it, and his sad old bathrobe with his chest showing, and that grippy smell of Vicks Nose Drops all over the place.
The potential confusion in the passage you quoted comes from the word "terrific" because this can be used to mean something amazing, as in "I'm the most amazing liar you ever saw in your life":
terrific adjective (VERY GOOD)
a terrific opportunity
You look terrific!
However, it also has two other potential meanings, both of which make more sense in the context of the quoted paragraph, although it's not entirely clear which. First, and most likely, it can also mean something in great quantity, and this is the sense it's meant in the quoted paragraph, as in "I'm the most prolific liar you ever saw in your life":
terrific adjective (VERY GREAT)
used to emphasize the great amount or degree of something:
The police car drove past at a terrific speed.
The less likely option is the original root of terrific from the root word terror, meaning something frightful or bad. Nowadays this is to some extent an archaic usage, which is why it's probably not what the author meant. However, it's still listed in some dictionaries and it does make sense in context as in "I'm the most reprehensible liar you ever saw in your life":
a: exciting or fit to excite fear or awe
a terrific thunderstorm
b: very bad : FRIGHTFUL
- Merriam-Webster Dictionary
The change in meaning was a slow process over the early 20th century whereby "terrific" came to mean not just "frightful" but a more general "intense". From there it's not such a big step for it to mean "intensely good". A less extreme case can be seen happening currently for "awful" as in "that's awfully good". Catcher in the Rye was serialised from 1945 onward before being published in 1951, and so would have been written after this drift in meaning had taken place.