Is bathing at night a good custom (sadhachara)?
There seems a translation issue if we read Sanskrit verse and other related verse in the same chapter/section of Mahabharata.
The sanskrit verse is
नक्तं न कुर्यात पित्र्याणि भुक्त्वा चैव परसाधनम
पानीयस्य करिया नक्तं न कार्या भूतिम इच्छता
And this is how it's translated (a part is mentioned in your question too)
One should never perform any act in honour of the Pitris at night time. One should not deck one's person after finishing one's meals. One should bathe at night, if one desires one's own advancement.
But, "One should bathe at night" is "One should not bathe at night" if we read Sanskrit verse. This is clear in Sanskrit-Hindi version:
You can also read in the same source (i.e. Mahabharata: Anusasana Parva: Section CIV):
न नग्नः कर्हि चित सनायान न निशायां कदा चन
One should never bathe in a state of nudity, nor at night.
संध्यां न भुञ्जेन न सनायान न पुरीषं समुत्सृजेत
परयतश च भवेत तस्यां न च किं चित समाचरेत
One should, O Bharata, carefully abstain from studying or reciting the Vedas, and eating, and bathing, at eventide. When the evening twilight comes, one should collect one's senses for meditation, without doing any act.