I just read C.S. Lewis's An Experiment in Criticism, and I thought it would be fun to adapt it to typography:
Good fonts are those which permit, invite, or compel good design. The function of typographic criticism is to multiply, prolong, and safeguard experiences of good design.
If I say I like Helvetica, and you are sure that it is bad, then you could say my taste is bad. On what do you base your position? If it is your own taste, then you are just being insolent. If you say so because of the prevailing view of the typographic community, then how long will this last? It is only a matter of time before fashion changes.
Suppose you had a different way to approach it. You might encourage me to talk about Helvetica, and discover that I have the worst sort of attitudes about design and typography (laziness, trendiness, etc). Suppose you then went around to other designers, and found that everyone who likes Helvetica shared the same bad attitudes. You would have solid ground for a steadily growing conviction that Helvetica is bad, arguing that 'Since all who enjoy Helvetica do so by applying the worst sort of design, Helvetica is probably a bad typeface'.
This approach has three advantages: first, it fixes our attention on the act of design in operation rather than abstract considerations. Second, basing judgment on how someone designs is much stronger than basing it on what typefaces they choose, which is subject to fashion. Finally, this method makes critical condemnation a very laborious task, which is a good thing (it is too easy otherwise).
This method relies on separating true typophiles from the general public. However, if you can find even one type lover who is fanatical about a given typeface, who would notice if there were a change, who would spend money to replace it if necessary, then you would need to exercise great caution before condemning it. Helvetica, under this method, could never be called a bad typeface. However, Comic Sans and Cheltenham (aka 'Chelt') might be.