As a spectral entity, Momma Bosco isn't confined to whatever appearance she was forced to have when she was alive. She could probably look like whatever she wants, but she's a self-confident woman content with how she looks. (Although why not make yourself look a bit younger?)
Alternately: It's Sam and Max! Very few things about their world make sense, and looking too much into it isn't good for you.
In other media and in studies of the metaphysical, ghosts are able to change their appearance for varying reasons. Some say they can change by will while others say changing depends on the ghost's mentality (if it acts like a child, it will appear like a child). Sometimes it may depend on who sees the ghost as it may take the appearance of how the watcher remember the deceased. I rather liked how Doug Tennaple viewed ghosts in his new graphic novel "Ghostopolis".
My explanation for Mama Bosco... Maybe she was being cooped up upstairs of the Inconvenience Store and was unable to do what she loved to do: invent and all that science jazz. Hence, when she died, her ghost still felt old and useless. But once she was able to get herself a laboratory and start working again in Season 3, she felt young again and so her ghost changed.
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Alternately: It's Sam and Max! Very few things about their world make sense, and looking too much into it isn't good for you.
My explanation for Mama Bosco... Maybe she was being cooped up upstairs of the Inconvenience Store and was unable to do what she loved to do: invent and all that science jazz. Hence, when she died, her ghost still felt old and useless. But once she was able to get herself a laboratory and start working again in Season 3, she felt young again and so her ghost changed.