What is good about Mr. Avery?

Mr. Avery is a minor character, showing up sporadically throughout To Kill a Mockingbird and usually not involved in overly important scenes. However, many of the scenes he is involved with add humor to the text. One of these memorable scenes where we see Mr. Avery is early in chapter 6, When Scout and Jem are going to sit with Dill by Miss Rachel's fish pond. The kids are spending their final evening with Dill before he has to return to his home (since summer has come to an end), and Scout asks,

"Reckon we better watch for Mr. Avery?" (chapter 6). 

Since nothing overly interesting typically happens in small-town Maycomb, the kids look for entertainment in simple things like watching Mr. Avery, who is known to sit "on the porch every night until nine o'clock and sneeze" (chapter 6). But this night, when they look over at his porch, they notice an

"arc of water descending from the leaves and splashing in the yellow circle of the street light, some ten feet from source to earth, it seemed to us" (chapter 6).

The source of the water is explained as Dill comments that "he must drink a gallon a day," and the two boys begin a contest "to determine relative distances and respective prowess" that leaves Scout feeling "left out again, as [she] was untalented in this area" (chapter 6).
While this scene is humorous, it shows how uneventful Mr. Avery's daily life is. The most exciting things the kids witness him do are relieve himself and sneeze. (But, their town isn't an overly exciting place at this time in the novel, and they choose to watch him sneezing on his porch.) Mr. Avery simply lives a pretty boring life. That is, until Miss Maudie's house catches on fire:

"Whose is it?"
"Miss Maudie's, hon," said Atticus gently.
At the front door, we saw fire spewing from Miss Maudie's dining-room windows. As if to confirm what we saw, the town fire siren wailed up the scale to a treble pitch and remained there, screaming. (Chapter 8)

Here, in a moment of town drama, Mr. Avery joins many other men to help salvage Miss Maudie's belongings:

The men of Maycomb, in all degrees of dress and undress, took furniture from Miss Maudie's house to a yard across the street. I saw Atticus carrying Miss Maudie's heavy oak rocking chair, and thought it sensible of him to save what she valued most.
Sometimes we heard shouts. Then Mr. Avery's face appeared in an upstairs window. He pushed a mattress out the window into the street and threw down furniture until men shouted, "Come down from there. Dick! The stairs are going! Get outta there, Mr. Avery!" (Chapter 8)

In this moment of danger, Mr. Avery risks his life to help Miss Maudie save her possessions from the fire. He proves a town hero, staying in the burning house after the other men have left and until the staircase begins to crumble. As he climbs out the upstairs window, his large body gets stuck in a window, causing the town to grow fearful, but soon later he gets free and is able to get himself out of the building safely.
Though Mr. Avery is not the most significant or interesting character in the novel, his willingness to help Miss Maudie in her time of great need shows his goodness and concern for others.
https://education.seattlepi.com/mr-avery-redeems-him-character-to-kill-mockingbird-5630.html