Segnosaur
Penultimate Amazing
First of all, I assume you are talking about a hypothetical "democratic majority", since they are obviously in the minority in congress.It would be kind of messed up if Mueller exonerated Trump but a democratic majority went for impeachment.
Secondly, it wouldn't exactly be "messed up", depending on what basis was used for impeachment. Mueller is investigating one main aspect... Trump and his Russian ties. However, there are other areas where impeachment might be possible, most notably the emolument's clause. This clause prevents a president from profiting from foreign governments, something that trump is accused of due to his current ownership of various properties (including Trump hotels that host foreign guests.) The emolument clause is a separate investigation and as far as I know, not being investigated by Mueller at all.
https://www.cnbc.com/2017/06/12/emoluments-clause-trump-accused-of-violating-constitution.html