This is from a Manga, and I don't quite understand how the soredake in the 2nd bubble fits into it? But maybe I am completely off with the meaning of the sentences as a whole - so I've put what I think they mean in italics as well.
Bubble 1:
俺はどっちかって
いうと
そういうのは
鈍い方だけど
I'd say I am a reserved (quiet/solemn) person, but
Bubble 2:
それだけ 俺も
航を見てたって
ことだ
I also watched Kou
Does the soredake simply add an "only"? i.e. I also only watched Kou.
Any pointers are much appreciated. Thank you!
「それだけ + Phrase or Mini-Sentence + って/という + こと（だ/です）」
would need to be remembered as a set phrase because it is a very common one. I means:
"That is/proves (just) how much ~~~~."
Thus, the sentence in question means:
"That is (just) how closely I was watching Kou."
Though there is not enough context provided, I would highly doubt that 「鈍{にぶ}い」 means "reserved, quiet, or solemn" here as you stated. I would think it is more like "slow" or "dull-witted".
Finally, regarding your translation of 「も」....
I will not go into details as it is not part of your question, but you will need to learn not to use "too" or "also" in your translation every time you see 「も」 because that is not what it always means. 「も」 is an extremely nuanced particle.
Bubble 1: The speeker confessed that he (or she) is not so sensitive to something (I guess other's emotion).
(I'm not sure what is the main subject in the cartoon, but I gess this even insencitive person might has noticed something that is the main subject related "航(a boy?)". This cartoon is a love story?)
Then,
Bubble 2: I guess, the meaning "それだけ" is the following,
Even insencitive person like me have noticed the internal something of "航," because I watched him (or her) more carfully than usual.
（何かに気が付くだけ（それだけ）、俺も航を見ていた）
