In both cases, it is a partitive article. It is needed when you mention an indefinite amount or quantity of an uncountable item. It has no equivalent in English: depending on the context, it may be translated as some, any, or would be left out.

Je mange de la viande

Je bois de l’eau

Je fais du pain

In your 2 cases, even though these are not strictly speaking items you can count or measure, it is the same idea.

C’est du mauvais français expresses the idea that is some "piece" of bad French.
In the same way, you could say

c’est de la mauvaise nourriture

c’est de la mauvaise musique

Je fais du tennis is more idiomatic: when you do an activity and you use the verb faire, in French, this activity is seen as an uncountable item that you are producing:

je fais du tennis

je fais de la peinture

je fais du bricolage

je fais du shopping