المسالة مهمة.

A natural Egyptian-style pronunciation is:

el-mas'ala mohimma

A few notes:

• ال is usually pronounced el- in Egyptian, not al-.
• مسألة / مسالة is roughly mas'ala. The apostrophe here shows a small throat-stop sound called hamza.
• مهمة is commonly pronounced mohimma in Egyptian speech.

A rough stress pattern is:

el-MAS-'a-la mo-HIM-ma

Because Arabic usually drops the verb to be in the present tense.

So Arabic says:

المسألة مهمة
literally: the issue important

But it means the issue is important.

If you want was, will be, and so on, Arabic does use a verb:

• المسألة كانت مهمة = the issue was important

This is very normal in both Standard Arabic and Egyptian.

ال is the Arabic definite article, meaning the.

So:

• مسألة = an issue / a matter
• المسألة = the issue / the matter

In Egyptian, ال is usually pronounced el-.

Here it stays clearly pronounced because م is a moon letter, so you say:

el-mas'ala

If the next letter were a sun letter, the l sound would blend into it instead.

In Arabic, descriptive words normally come after the noun, not before it.

So Arabic patterns like this:

• بيت كبير = a big house
• بنت جميلة = a beautiful girl

In your sentence, مهمة comes after المسألة, which is the normal Arabic order.

Because مهمة here is not an adjective directly attached to the noun in the sense of the important issue. It is the predicate of the sentence:

المسألة مهمة = the issue is important

If you said:

المسألة المهمة

that would mean:

the important issue

So the difference is:

• المسألة مهمة = a full sentence: The issue is important
• المسألة المهمة = a noun phrase: the important issue

This is one of the most important differences for learners to notice.

Because المسألة is a feminine noun, and adjectives in Arabic agree with the nouns they describe or refer to.

So:

• feminine noun → feminine adjective
• المسألة مهمة

You can see the feminine ending ـة on both words.

Compare with a masculine noun:

• الموضوع مهم = the topic is important

Also, the final ـة is usually pronounced -a at the end of a word in normal speech, so:

• مهمة sounds like mohimma not mohimmat in this sentence

In careful standard spelling, المسألة is the usual form, with a hamza:

المسألة

You may also see:

المسالة

especially in informal writing, texting, or less careful spelling.

So:

• المسألة = standard/careful spelling
• المسالة = common informal simplification

For a learner, it is good to recognize both, but it is safest to learn the standard form المسألة.

It is understandable and correct, but it sounds a bit formal or neutral, not especially casual.

In everyday Egyptian, people might also say something like:

• الموضوع مهم = the topic / matter is important
• الموضوع ده مهم = this matter is important

Still, المسألة مهمة is perfectly good Arabic, and Egyptians would understand it easily. It just leans a little more toward formal or written-style language than very casual street Egyptian.