There is a small stone on a flat surface like a table and Tom puts his mug on it as shown in the picture.
Is it correct to say "The mug's wobbly if you do like that"?
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"if you do it like that" would be fine - "like that" refers to the way something is being done, it implies that there are different options, and Tom's chosen to do "it" (the act of putting a cup down) in a bad way. "Don't do it in that way, do it like this".
"If you do that" just refers to the action - the thing you're doing will have this effect. So don't do that thing!