First, Ucchi is definitely not generally gay, don't be contrarian for contrarian's sake. She is only interested in Tomoko.
Chapter where she gives Tomoko omiyage. Blushes deeply when she receives a thank you letter.
Chapter where they exchange chocolates. When Tomoko leaves hers in Ucchi's locker, she silently holds back. Not a joke (though this chapter does end on a joke of her eating poop)
Omake where she notices that Tomoko's nails are painted. Ends with her sad, not a joke.
Ucchi spicing up Tomoko's food at a party and blushing. No joke there.
Naturally, the chapter where Ucchi shoots herself in the foot. Almost the entire chapter is taken seriously, and a joke is only made when her being closet gay gets so bad for her that she pretty much ruins her already fragile relationship with Tomoko.
Are most of Ucchi's moments comedic? Yes. Are all of them? Definitely not, and there are moments like post-Amusement Park where it's a blend of Ucchi taking things seriously and Ucchi being made fun of (getting the key chain and really feeling sincere about having received a special gift, only to realize Tomoko is getting away from her and she is NOT special).