I found a better solution than ignoring the message. I was able to stop getting it by making Schema happy.
When I imported a table from Access to MySQL and then adopted it for the Data module ("Structure > Data tables > Adopt tables") I got the same message for two table fields, one that was a "Yes/No" in Access and the other that was a "Date/Time". The importation from Access had converted those fields to types "bit(1)" and "datetime", respectively, in MySQL. Drupal's data schema does not have bit types or temporal (date/time) types, so the Schema module issued those messages.
My solution was to convert those table fields to types that are consistent both with Drupal's schema and with the data in my table. I converted the "bit(1)" field to type "tinyint(1)" and the "datetime" field to type "double". (The "datetime" data type is the same as the "double" type except that it's designated to represent a date and time.)
I checked that the conversion had not changed the values of the data in the table. For the datetime field, this was done using the SQL function "convert(<field name>, datetime)". The check was successful; the before and after values were the same.
Then I went back to "Data tables" and adopted the table with the converted data types. No more complaints from Schema!