Nothing wrong with that, particularly if those opinions are communicated in a way that makes them look like statements of fact.
Someone being able to say "I think your opinion is wrong" is no less a freedom of speech matter than someone being able to state an opinion in the first place. Freedom of speech does not, or at least it should not, give special privilege or protection to the first person who speaks.
This is a tough position for Twitter because they now have to fact check practically all of his tweets. Any tweet not checked will be seen either as tacit endorsement by Trump's political opponents or 'undeniable truth' by some portion of their users regardless of validity.