、。
Tanabata comes up often in manga, so I know about it to a reasonable extent.
Literal
Tanabata [topic-は] manga [in-に] often appears-[come-てくる][because-ので], I [also-も] moderately know-[continuous-polite-ています].
出てくる ('to come out, to appear') uses ~てくる to mark motion toward the speaker's perspective — here 'to show up (in a story).' そこそこ is an adverb meaning 'to a moderate/reasonable degree, fair-to-middling' — neither a little nor a lot. 七夕 (Tanabata) is the 'Star Festival' celebrated on July 7, based on the legend of the cowherd Hikoboshi and weaver Orihime. ~ので 'because' is slightly more polite than から.